On 2003 September 16-18, NASA's Marshall Space Flight Center (MSFC) hosted the symposium 4 Years of Chandra Observations: A Tribute to Riccardo Giacconi. Over 150 attendees participated in the Symposium, which featured over 50 oral and 80 poster presentations. Symposium honoree Riccardo Giacconi, 2002 Nobel Laureate in Physics, delivered the keynote address.
Here we present abstracts for all the Symposium's scientific presentations, organized by topic. For approximately half the presentations, the abstract's title provides a link to the corresponding (PDF) presentation, reproduced with permission of the respective lead author.
|0 KEYNOTE ADDRESS||8 GAMMA-RAY BURSTS|
|1 SOLAR SYSTEM||9 GALACTIC CENTER|
|2 STELLAR CORONAE||10 CLUSTERS OF GALAXIES|
|3 SUPERNOVA REMNANTS||11 INTERGALACTIC MEDIUM|
|4 INTERSTELLAR MEDIUM & STELLAR WINDS||12 GALAXIES|
|5 NEUTRON STARS||13 ACTIVE GALACTIC NUCLEI|
|6 GLOBULAR CLUSTERS||14 EXTRAGALACTIC SURVEYS|
|7 BLACK HOLES & ACCRETION DISKS||15 Chandra INSTRUMENTS|
R. Giacconi (JHU & AUI)
By the 1960s, space technologies enabled the first x-ray observations of extra-solar sources. From early rocket experiments, to large-area detectors on orbiting satellites such as Uhuru, to focusing orbital telescopes such as Einstein (HEAO-2), to the high-resolution optics of Chandra, x-ray sensitivity has improved by over nine orders of magnitude. This advance has profoundly influenced astronomy, allowing x-ray studies for virtually all categories of celestial objects from planets to normal stars, from ordinary galaxies to quasars, from small groups of galaxies to the farthest known clusters. These studies demonstrate the fundamental role of high-energy phenomena in the formation and in the chemical and dynamical evolution of structures on all scales. X-ray observations have proved crucial in discovering important aspects of these phenomenaBack to Topics
J.J. Drake (SAO), B.J. Wargelin, D.A. Swartz, & S.J. Wolk
The elements Mg, Al, and Si dominate the crustal and mantle mineralogy of the lunar surface and knowledge of their surface distribution is important for understanding lunar evolution. To date, the few Apollo site samples and very limited x-ray maps provide our only information on the abundance distributions of these elements. The composition of the lunar surface can be probed remotely by x-ray observatories through analysis of fluorescent lines excited by incident solar x rays. Such studies could greatly expand our understanding of lunar geochemistry at low cost and on a very short timescale compared to future lunar spacecraft. We present here an analysis of Chandra ACIS-I calibration observations of the moon obtained at such a phase that both dark and illuminated hemispheres are visible in the ACIS image. Despite the unfavorable illumination geometry and short exposure times, we are able to obtain some new information on the relative Mg, Al, and Si abundances. These results are discussed in the context of lunar evolution, and we also examine the prospects for exploring the mineralogy of other rocky solar system bodies, such as the asteroids, through similar remote sensing studies with future generation x-ray observatories.
H. Tsunemi (Osaka), K. Mori, H. Katayama, D.N. Burrows, G.P. Garmire, & A.L. Metzger
On 2003 January 5, Saturn passed across the x-ray bright region of the Crab Nebula, the first occurrence since the birth of the nebula in 1054. Because the Crab Nebula is one of the brightest synchrotron sources in the sky, we can study the object placed in front of it just as we usually do in the laboratory. Due to the radiation-zone passage of Chandra, we only observed the transit by Saturn's largest satellite, Titan. One of the biggest satellites in the solar system, Titans diameter is 5150 km subtending about 1 from the Earth. Chandra is the only x-ray satellite to resolve Titan. Titan is also known to have an atmosphere 1.5 times thicker than that of the Earth. The radio, IR, and optical observations by Voyager 1 in 1980 probed the thermal structure of Titan's atmosphere but no x-ray observation has been done so far. The observation was planned to maximize the number of x-ray photons around Titan. We clearly detect an occultation shadow and find that it is larger than the corresponding extent of the Titan's solid surface. The difference gives a thickness of Titan's atmosphere of 88264 km for x-ray absorption. This value is slightly larger than those estimated from previous Voyager results, suggesting a temporal variation. The Cassini/Huygens mission will perform a detailed observation of Titan in 2005.
R.F. Elsner (NASA/MSFC), R. Gladstone, H. Waite, N. Lugaz, P. Ford, T. Cravens, R. Howell, D. Grodent, G. Branduardi-Raymont, P. Rodriguez, & A. Bhardwaj
The Chandra X-ray Observatory observed Jupiter in late 2003 February for 144 ks, using both the ACIS-S and HRC-I imaging x-ray cameras. Five orbits of HST STIS observations of the planet's northern auroral zone were obtained during the ACIS-S observations. Jupiter's emission and that of the Io plasma torus were weaker than in our previous observations in 2000 December. The new data are providing a wealth of information about Jupiter's auroral activity, including the first x-ray spectra from the x-ray hot spots inside the auroral ovals. These spectra are consistent with line emission from highly charged states of sulfur and oxygen. Although irregular time variability, with a timescale of about 30 minutes, is observed in these data, the fairly regular 45-minute quasi-periodicity in auroral x-ray emission observed with the HRC-I in 2000 December is not obviously present. There is a rotational phase relation between the emission from the northern and southern x-ray aurora. We look forward to comparison of the Chandra results with recent and planned XMM-Newton observations of Jupiter.
B.J. Wargelin (SAO), M. Markevitch, M. Juda, V. Kharchenko, R.J. Edgar, & A. Dalgarno
We have analyzed data from two sets of calibration observations of the Moon made by Chandra, clearly detecting time-variable soft-x-ray emission, primarily O VII Ka and O VIII Lya, when viewing the optically dark side. The ROSAT observatory previously detected such emission, which was tentatively ascribed to energetic solar-wind electrons impacting the lunar surface. The Chandra observations, however, with their better spectral resolution, strongly favor solar-wind charge transfer with neutral hydrogen in the Earth's geocorona as the mechanism for this emission. We theoretically model the geocoronal emission and show that the predicted spectrum and intensity match the Chandra and ROSAT observations well. We also model the closely related process of heliospheric charge transfer and find that the total charge-transfer flux observed from low-Earth orbit amounts to a significant fraction of the soft x-ray background emission measured by ROSAT. This work was supported by NASA contract NAS8-39073 to the Chandra X-ray Center and by NASA's Space Astrophysics and Analysis program under grant NAG5-10443 (BJW).Back to Topics
J.P. Cassinelli (Wisconsin), W.L. Waldron, & N.A. Miller
Chandra spectra have provided spectrally resolved line profiles and this has greatly improved our ability to diagnose the x-ray formation regions in the outer envelopes and winds of early-type stars. With the resolved FIR lines of helium-like ions, we are able to determine at what radius the x-ray lines are originating. Although the radii differ drastically from one ion to another, the source location occurs at about optical depth unity in the wind. This is as expected from models in which shock fragments are distributed throughout the winds. However, rather unexpected is the information that high ion stages come from regions near the base of the wind where the wind speeds and associated shocks should be too weak to produce the high ion stages observed. For most hot stars, the x-ray lines show negligible centroid shifts and unexpectedly minor skewness. Various new ideas are being supported such as x rays that form in bow-shock structures in the winds, or in shocks at the interfaces of the winds and circumstellar equatorial disks.
E.D. Feigelson (Penn State)
In 1972, Giacconi and colleagues tentatively identified 2U 0525-06 with the Orion Nebula. This inaugurated a sub-field of x-ray astronomy demonstrating that ordinary stars exhibit their highest levels of magnetic reconnection activity during their earliest phases of evolution. Thousands of pre-main-sequence stars many previously unknown are now being studied with Chandra. We present some recent results here, highlighting two topics. (1) A vast range of flare characteristics is seen in young stars. Luminosities range up to Lx 1032 erg s-1, 104 above the strongest contemporary solar flares. Time scales range from minutes to days, some showing standard cooling decays and others not. Elemental-abundance anomalies in the x-ray plasma are common. (2) The activityrotation relation, which accurately accounts for the 103 range in main-sequence stellar x-ray emission, is completely absent in pre-main-sequence stars. Instead, x rays are linked to a combination of stellar mass and size. This may be the first observational evidence for a distributed, turbulent magnetic dynamo in deeply convective stars.
S. Sciortino (INAF/Palermo), F. Damiani, E. Flaccomio, G. Micela, F.R. Harnden Jr., & S.S. Murray
In a deep (60-ks) Chandra ACIS observation of the very young star cluster NGC 6530, we detect 884 x-ray point sources and argue that a very large fraction of them (90-95%) must be pre-main-sequence cluster members, mostly of low masses. This is a significant enlargement of the known NGC 6530 stellar population with respect to previous optical studies, including Ha surveys. We identify 220 x-ray sources with catalogued stars down to V = 17, while most unidentified sources have fainter counterparts. The identified cluster x-ray sources are found in a band in the H-R diagram above the main sequence, in the locus of 0.51.5-Myr pre-main-sequence stars, with masses down to 0.51.5 Msun. We find evidence of an age gradient across the field from northwest to south, suggesting a sequence of star-formation events qualitatively similar to that found in earlier studies of the same region, but differing in the details. A group of x-ray sources showing frequent flares may be associated with the youngest stars in the cluster, suggesting that x-ray-flaring activity is especially intense in the youngest pre-main-sequence phases of low-mass stars.
J. Rho (CalTech), S.V. Ramirez, M.F. Corcoran, K. Hamaguchi, & B. Lefloch
The Trifid Nebula, one of the youngest star-forming HII regions, was observed for 16 hours by the ACIS-I detector on board of the Chandra X-ray Observatory. We detected 304 x-ray sources, 30% of which are hard sources, with near-infrared counterparts for two-thirds of the x-ray sources. Chandra resolved the HD164492 multiple system into a number of discrete x-ray sources. X-ray emission is detected from components HD164492A (an O7.5III star which ionizes the nebula), B, C (a B6V star), and D (a Be star). Components C and D are blended and together have comparable x-ray brightness to the O star. HD164492A has a soft x-ray spectrum (kT 0.5 keV), while the component CD blend shows much harder x-ray spectrum (kT 6 keV). The CD blend and other hard sources are responsible for the hard emission and Fe-Ka line seen by the ASCA, which was previously attributed entirely to HD 164492A. The soft x-ray spectrum of the O star is similar to emission seen from other single O stars and is probably produced by shocks within its massive stellar wind. Lack of hard emission suggests that neither a magnetically confined wind shock nor colliding wind emission is important in HD164492A. In contrast, the hard emission and high x-ray luminosity, if attributed entirely to the Be star (HD164492D), may imply magnetic activity. A dozen variable stars are found in the field; most of them are pre-main-sequence stars and some older OB stars show significant, but low-level variability. We identify pre-main-sequence stars from those variables that show fast rise and slow-exponential-decay light curves similar to x-ray light curves of other pre-main-sequence stars, and/or those that have counterparts identified as TTS and YSO using near-infrared colors. The spectra of variables with near-infrared or optical counterparts show temperatures of 14 keV and luminosities of 0.92.51031 erg s-1, which are comparable to Class II and III (and possibly Class I) objects. Among the variables, 3 had no near-IR, optical, radio counterparts, and their light curves show an extreme contrast of x-ray flux between the flare and quiescent stages. We suggest these x-ray-only detected variable sources are in an early stage of the pre main sequence, possibly Class I or earlier. Interestingly, we also detected x-ray counterparts from two massive star-forming cores with bipolar wings and associated Class-0 candidates. Their spectra show high extinction toward these sources and imply a high luminosity of 251031 erg s-1. It is unclear if the x-ray emission of Class-0 objects is due to solar-type magnetic activity, as in Class-I objects; thus, we discuss a few alternatives.
J.L. Linsky (JILA/Colorado), A. Brown, & J. Brown
We analyze Chandra High-Energy Transmission-Grating Spectrometer (HETGS) spectra of the M dwarf stars AU Microscopii (dM 1e) and AD Leonis (M3.5 V). AU Mic is the x-ray brightest M dwarf; AD Leo is a well-known flare star. Since Chandra detected no large flares during our observations, we derive quiescent emission-measure distributions using data from the HETGS and from earlier EUVE spectra. We discuss abundances, electron densities, and heating mechanisms for the coronae of these two stars.
J.L. Linsky (JILA/Colorado) & T. VanVlliet
We describe and analyze coordinated Chandra ACIS-S, HST STIS, and FUSE observations of the M5.5 Ve flare star Proxima Centauri (Alpha Cen C, GJ 551). The observing program includes three sessions with the Chandra ACIS-S, one of which includes the HETGS, and one session with the HST STIS with the E140M grating. The FUSE observation was taken several days later. The first Chandra observation is simultaneous with the HST observation. Since the star was observed in the continuous viewing zone, we have long, uninterrupted observing sequences. We observe Prox Cen in quiescence and during several small and moderate-sized flares. We derive an emission measure distribution for quiescence, and discuss the change in the emission measure distribution during the flares. We model the moderate-sized flare as a large loop, infer its physical properties, and discuss the applicability of the 2-Ribbon solar-flare model to stellar astronomy.
J.L. Linsky (JILA/Colorado), A. Mytyk, M. Gagne, M. McCaughrean, & M. Andersen
We present the first x-ray images ever obtained of the Eagle Nebula star-forming region. On 2001 July 31, the Chandra X-ray Observatory obtained a 78-ks image of the Eagle Nebula, the dark columns of dust and cold molecular gas in M 16, and the 2-Myr cluster NGC 6611, revealing 1103 x-ray sources in the 1717 ACIS-I field-of-view. Of these x-ray sources, 959.have optical or near-IR counterparts. No x-ray sources are associated with the evaporating gaseous globules (EGGs) first observed in HST WFPC2 images of M16 by Hester et al. (1996). Chandra detected all 11 O stars, 22 of 39 B0B2 stars, and 5 of 13 late-B stars in NGC 6611.
E.D. Feigelson (Penn State)
Planets form in cold molecular and dusty disks around pre-main-sequence stars, which are mainly studied in the IR/mm bands, and in primitive solar-system bodies. I argue here that x-ray astronomy can provide valuable, often unique, insights into disk evolution and planet formation. (1) Young stellar x rays penetrate to surprising depths into the disk, producing sufficient ionization to induce the BalbusHawley instability and turbulence, and ionize the base of bipolar outflows. (2) The presence of daughter isotopes of 10Be, 41Ca, etc. in pristine meteorites is often attributed to injection of material from a nearby supernova. Chandra observations of young solar analogs quantitatively support the alternative model of in-situ spallation by high fluences of MeV particles from magnetic-reconnection flares. (3) Another mystery has been the origin of meteoritic chondrules. One possibility is flash melting by UV/x-ray radiation from flares. (4) A variety of other x-ray effects on disk material have been suggested recently heating outer molecular-disk layers, non-equilibrium chemical reactions, sputtering of disk grains, and melting of dust mantles. (5) A critical input into planet formation is the longevity distribution of circumstellar disks, which in turn depends on x-ray selected samples of older (around 10 Myr) pre-main-sequence stars.
J.J. Drake (SAO) & M.J. Sarna
Chandra Low-Energy Transmission Grating Spectrograph (LETGS) observations of the pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance ratio of the K-dwarf component for the first time. While the white-dwarf component dominates the spectrum longward of 50 � at shorter wavelengths the observed x-ray emission is entirely due to coronal emission from the K dwarf. The H-like 2p 2P3/2,1/2 1s 2S1/2 resonance lines of C and N yield an estimate of their logarithmic abundance ratio relative to the Sun of [C/N] = -0.380.15 about half the currently accepted solar value. We interpret this result as the first clear observational evidence for the presumed common-envelope phase of this system, during which the surface of the K dwarf was contaminated by CN-cycle processed material dredged up into the red-giant envelope. We use the measured C/N ratio to deduce that the K dwarf accreted 0.0150.04 Msun while engulfed. This is consistent with a recent tentative detection of 13C in the K-dwarf photosphere and with the measured Li abundance in the scenario where the red-giant companion was Li-rich during the common-envelope phase.
M. Tsujimoto (Penn State), K. Koyama, K. Imanishi, & Y. Tsuboi
We observed the Orion molecular cloud 2 and 3 using the Chandra X-ray Observatory and detected 385 x-ray sources. We also observed the same region in J, H, and K bands using the University of Hawaii 88-inch telescope. Combining the x-ray and NIR data, we identified NIR counterparts to 278 x-ray sources. Here we report the result of spectral and temporal analyses of the NIR-identified x-ray sources. We focus on the 142 brightest sources, with more than 50 x-ray counts and a S/N larger than 10. We first fitted the spectra with a one-temperature plasma model. If the one-temperature fit was statistically unacceptable, we tried a two-temperature model. For sources with flare-like variability, we also conducted time-sliced spectral analysis for the flare and quiescent phases. We found that (1) the temperature histogram has two peaks at 1 keV and 23 keV, (2) most of the sources with a two-temperature plasma show the combination of 1 keV and 23 keV, (3) the two-temperature plasma can be seen both in flare and in quiescent phases, and (4) the higher temperature plasma increases its emission measure in flare phases. These results suggest that pre-main-sequence sources have two different x-ray emission mechanisms that are represented by high- and low-temperature plasmas. In comparison with the Sun and other main-sequence sources, we discuss that these emissions are from the flare and the coronae, respectively. This presentation is based in part on a Ph.D. thesis (MT) available at http://www.astro.psu.edu/users/tsujimot/english/materials/phd.pdf.
W.L. Waldron (EER Systems) & J.P. Cassinelli
The distribution of x-ray temperatures in OB stellar winds allows us to establish strong constraints on the x-ray production mechanism. Using Chandra HETG spectra from 11 OB stars, we determine the x-ray temperatures, emission measures, and radial locations for all observed H-like and He-like emission lines and for two strong Fe XVII emission lines. We find that all OB x-ray temperature distributions indicate a decreasing x-ray temperature structure outward from the stellar surface. Although the low x-ray temperatures (< 5 MK) in the outer wind regions are compatible with a shock model, the deeply embedded (< 2 stellar radii) higher x-ray temperatures are difficult to understand in terms of current shock-model predictions. The x-ray density distributions are also found to be peculiar, where in many cases, the x-ray densities are significantly larger than expected. The relationship between the x-ray-line emission measures (EM) and temperatures (T) for OB stars display three clearly different dependencies for log(EM) ~ n log(T) n < 0, n > 0, and n 0. We discuss the consequences of these results.
V.L. Kashyap (SAO), N. Evans, A. Mossman, D. Turner-Bey, J.J. Drake, P. Green, F.R. Harnden Jr., D.W. Kim, B. Wilkes, & ChaMP collaboration
We present preliminary results from the Chandra Extended Stellar Survey (ChESS), part of the ongoing Chandra Multi-wavelength Project (ChaMP). We have searched for optical matches to x-ray sources serendipitously detected in ChaMP, using the GSC, USNO, Tycho, and ChaMP-optical-follow-up catalogs. We present the first catalog of serendipitously detected stellar x-ray source candidates and discuss the x-ray properties of this sample of stars.
D.A. Swartz (USRA/NSSTC), J.J. Drake, R.F. Elsner, K.K. Ghosh, & A.F. Tennant
The nearby Herbig Ae star HD 163296 has a strong, possibly x-ray-emitting, jet extending 6 above its accretion disk. The combination of an accretion disk and jet in a relatively old Herbig Ae object, a low extinction due to a lack of association with dark clouds, the presence of HerbigHaro objects along the jet axis, and an otherwise source-free field make HD 163296 a particularly promising object for imaging spectroscopy with Chandra. We present results of a recent ACIS-S imaging observation of HD 163296 designed to image the jet, disk, and other structures, and to probe the x-ray emission mechanism through broadband spectroscopy.
S.A. Drake (NASA/GSFC), M. Audard, T. Simon, K.P. Singh, & M. Guedel
V824 Ara (HD 155555) is an interesting nearby example of a young (ZAMS or Pre-MS) tidally locked dwarf close-binary system. The orbital-rotational synchronism has induced a very high level of coronal activity in this system, making it an attractive target for high-resolution x-ray spectroscopy. We obtained a 95-ks HETG/ACIS-S observation of this close binary and its nearby visual binary dMe companion LDS 587 B that yielded a high signal-to-noise spectrum of the system. As we show, the basic characteristics of this spectrum are quite similar to those of evolved RS CVn binary systems, such as AR Lac and HR 1099. The pattern of inferred coronal abundances is also similar in showing the usual trend of increasing relative abundance with first ionization potential seen in other stars with similar activity levels. Thus, there is no evidence from the current data that coronal properties of stars at the saturation limit are different for young stars as compared to evolved stars.
K.V. Getman (Penn State), E.D. Feigelson, L.K. Townsley, G.P. Garmire, P.S. Broos, & Y. Tsuboi
We have obtained a 40-ks x-ray image of the high-mass star-forming region of M17 (Omega Nebula) with the Chandra ACIS-I. In addition to prominent diffuse soft-x-ray emission, our image, with a sensitivity log(Lx [erg s-1]) 29.7, revealed 889 x-ray sources. While 832 of these sources can be identified with cluster members and foreground stars known from JHK-band surveys, more than half the 57 unidentified objects are most likely new cluster members. The x-ray luminosity function of the detected young stars spans a range log(Lx [erg s-1]) 29.733.5 in the 0.58 keV band; the absorption column ranges from log(NH [cm-2]) 21.723.5. The x-ray light curves of several sources reveal powerful flares with parameters typical for x-ray-active young stellar objects (YSOs). We construct and analyze the x-ray spectra of the stronger sources and derive plasma temperatures between 0.7 keV and 5 keV for the T Tauri stars and deeply embedded YSOs in M17, and higher temperatures (up to 12 keV) for many flaring sources. We find that most of deeply embedded x-ray YSOs are distributed in or southwest of the ionization front excited by the central OB cluster. This supports the theory of induced star formation in the molecular cloud, most actively in the southern bar. We describe in detail the x-ray properties of the most interesting individual sources including the KW object, the ultra-compact HII region M17-UC1, and the protostellar object 182022.9-161152 deeply embedded within the dense molecular core.
K.V. Getman (Penn State), L.K. Townsley, E.D. Feigelson, P.S. Broos, M. Tsujimoto, G.P. Garmire, E. Flaccomio, S.J. Wolk, F.R. Harnden Jr., S.S. Murray, S. Sciortino, F. Damiani, G. Micela, J.H. Kastner, J. Li, & M.J. McCaughrean
The Chandra Orion Ultra-deep Project (COUP) combines 6 consecutive ACIS-I observations of the Orion Nebula Cluster obtained in 2003 January, with 0.84-Ms total exposure time. Over 1600 point sources are detected in this star-forming region, most showing variability in their light curves. We describe some of the data-analysis challenges specific to this observation source detection with complex backgrounds due to bright sources, resolving crowded sources, spectral fitting of sources lying under readout streaks, and time-varying pile-up. We show example output from the custom software we have developed to automate the spatial, spectral, and timing analysis of these sources and to collate the results into user-friendly viewing formats. Much of this software is available to the Chandra community now and more will be made public as the tools mature.
M. Orio (Wisconsin), S. Starrfield, & E. Tependenlegliolu
We present Chandra observations of the peculiar eruptive variable V838 Mon and of the recurrent nova IM Nor. In an ACIS-S observation about a year after its outburst, we failed to detect V838 Mon as an x-ray source. The upper limits we derive on the x-ray luminosity cause us to rule out an explanation of the eruption in terms of a nova-like thermonuclear event in a symbiotic system. We discuss other applicable physical models in light (or better, absence of light) of the Chandra observation. In an ACIS-S observation of the recurrent nova IM Nor, we did not detect an x-ray source 1 month after the outburst. However, we did detect a moderately hard and not very luminous source 3.5 months later. We discuss the x-ray behavior of this object and how it differs from other recurrent novae, giving important clues on the nature of the system and its physical parameters.Back to Topics
U. Hwang (NASA/GSFC)
Chandra and XMM-Newton have been providing new resources and a wealth of data to x-ray observers for the past four years. I give an overview of their significant contributions to the study of supernova remnants, highlighting recent results for young remnants in which x-ray emitting stellar debris are prominent. I discuss their use to study supernova nucleosynthesis and the various other known processes that can affect the distribution of the ejecta in remnants. I also discuss work to study the interactions of such remnants with their environments, their dynamics, nonthermal emission from shocks, and comparisons of multi-wavelength emissions.
J.P. Hughes (Rutgers)
I review selected results on supernova remnants that have been made possible by the superb imaging capabilities of the Chandra X-ray Observatory. Topics to be covered include studies of motions, nucleosynthesis, cosmic rays, and shock physics.
R.A. McCray (JILA/Colorado), S. Park, & D.N. Burrows
During the first 10 years after its initial outburst, the radiation from SN 1987A was dominated by energy deposited in the interior of the supernova debris by the decay of newly synthesized radioisotopes. Today, the blast wave from SN 1987A is overtaking the inner circumstellar ring, resulting in the appearance of many optical hot spots on the ring seen in HST images and spectra. With Chandra and the Australia Compact Telescope Array, we are also observing rapidly brightening rings of x-ray and nonthermal radio emission, respectively, from the same interaction. This event marks the birth of the supernova remnant, SNR 1987A, defined as the epoch when its light is dominated by the impact of the supernova debris with its circumstellar matter. The observations provide unique opportunities to understand the physics of the shock interaction and the structure of the supernova debris and the circumstellar matter. We describe what we have learned about SNR 1987A from Chandra and HST observations and theoretical modeling, and what we can expect to learn from future observations of this rapidly developing event.
P.O. Slane (SAO), D.J. Helfand, S.S. Murray, & E.V. Gotthelf
As the presumed remnant of SN 1181, 3C 58 houses one of the youngest-known neutron stars in the Galaxy. The properties of this young pulsar and its associated wind nebula differ considerably from those of the Crab, and may well offer a more typical example of the endpoint of massive-star collapse. While the luminosity is much lower than that for the Crab and the size is larger, there are several morphological similarities. Our Chandra observations reveal structures in the inner nebula of 3C58 that may be associated with the pulsar-wind termination shock, a jet that may be aligned with the rotation axis, and other regions of enhanced emission. Spectral variations in the pulsar-wind nebula (PWNe) are consistent with the expected evolution of the post-shock flow, and the location of the termination shock is consistent with pressure balance between the wind and the nebula. Limits on the neutron-star surface temperature fall below standard cooling models, indicating that some more rapid neutrino cooling process is required. Most recently, our deep Chandra observations reveal rich and complex structure in the interior of the nebula, as well as direct evidence of a thermal shell of ejecta-rich material bounding the PWNe.
K. Mori (Penn State), D.N. Burrows, G.G. Pavlov, S. Shibata, J.J. Hester, & H. Tsunemi
We present morphological variations of the Crab Nebula revealed by Chandra observations over the 3 years since its launch. In the previous Chandra symposium, we reported on short-term (days to weeks) variations seen in the inner Crab Nebula: Wisps are emerging from the inner ring and blobs are moving out along the jet. In addition to the short-term variations, we have now discovered long-term (years) variations: The circular structures previously seen northeast of the torus appear to have decayed and the kinked-structure of the southern jet has changed. We discuss the origins of these variations in terms of the pulsar-wind-nebula mechanism.
J.W. Keohane (CalTech), J. Rho, & K. Borkowski
We present a Chandra observation of the archetypical mixed-morphology supernova remnant (SNR) W28. The 89-ks observation was performed in the ACIS-S imaging mode (4 ACIS-S and 2 ACIS-I chips). W28 has a 50 diameter; our observation covered the central and southwestern parts of the remnant. While previous ASCA/ROSAT studies of W28 showed that the plasma conditions are different from other mixed-morphology SNRs, with spectral variations seen across the remnant, these Chandra data make this less clear by revealing hard point-like x-ray emission. This source is located on the I0 chip, 20 southwest of the center and on the SNR shell. The spectrum of this hard emission is well modeled by a power law, suggesting nonthermal emission. We discuss possible physical explanations, such as an associated synchrotron nebula or an unrelated source. In the center of the SNR, lies a bright x-ray region a few arcminutes in diameter, surrounded by fainter diffuse emission with filamentary structure. No strong spectral variations are found in the central region within one ACIS chip field of view. We discuss the high-resolution x-ray image of W28 in comparison to the optical emission, possible x-ray mechanisms for this center-filled x-ray emission, and implications for other mixed-morphology SNRs.
J.S. Warren (Rutgers), J.P. Hughes, & P.O. Slane
We present Chandra x-ray observations of the young supernova remnant (SNR) 0509-67.5 in the Large Magellanic Cloud (LMC) that is believed to be the product of a Type-Ia supernova (SN Ia). The remnant is very round in shape, with a clear clumpy shell-like structure. Our Chandra data reveal the remnant to be rich in silicon, sulfur, and iron. The yields of our spectral model fits confirm that 0509-67.5 is the remnant of an SN Ia and shows a clear preference for delayed detonation models. We find a single isolated knot enhanced in iron similar to one seen in Tycho's SNR, arguing for the presence of modest small-scale composition inhomogeneities in SN Ia's. Two cases for the continuum emission were modeled one where most of the electrons come from hydrogen and form the continuum, another where all of the electrons come from the partially ionized metals and the continuum is nonthermal in nature. The former case requires a relatively large value for the ambient density ( 1 cm-3). Another estimate of the ambient density comes from using the shell structure of the remnant in the context of dynamical models. This requires a low value for the density (< 0.1 cm-3), which is more consistent with other evidence. Therefore, we conclude that the bulk of the continuum emission from 0509-67.5 has a nonthermal origin.
D. Dewey (MIT), C.R. Canizares, K.A. Flanagan, A. Fredericks, J.C. Houck, M.S. Noble, & M.W. Wise
Chandra, with its high spatial and spectral resolution, provides details and clues that can give information on the 3-D structure of x-ray sources. In this work, we summarize what we've learned from Chandra about the supernova remnant E0102-72 in the Small Magellanic Cloud (SMC), including the following: progressive radial ionization in the reverse shocked ejecta; temperature variations in the remnant; an outer blast-wave component; and spatial-velocity structure suggestive of roughly cylindrical ejecta emission. We then create simple 3-D data structures to model these features of the remnant and derive relevant quantities. Some of the directions here may be useful in 3-D modeling of other x-ray systems, both resolved (e.g., clusters of galaxies) and unresolved (e.g., accretion-disk systems).
T.G. Pannuti (CalTech/JPL) & G.E. Allen
G266.2-1.2 (RX J0852.0-4622) is a member of the small but growing class of dynamically young, shell-type Galactic supernova remnants (SNRs) that feature x-ray spectra dominated by nonthermal emission. Such emission may be produced by synchrotron radiation emitted by high-energy electrons gyrating in the magnetic field of the SNR. We are currently conducting a detailed analysis of this type of emission as exhibited by several Galactic SNRs and exploring its relationship with cosmic-ray acceleration by SNRs. As part of this study, we present a 75-ks observation of the luminous northwestern rim of G266.2-1.2 made with the Chandra ACIS-S instrument. Fine structure in this rim including both a leading and trailing rim has been revealed for the first time by these observations. Initial results from the analysis of these data (including a search for thermal emission from this SNR) are presented and discussed. TGP acknowledges support from NASA LTSA grant NAG5-9237.
S.P. Hendrick (NC State), K.J. Borkowski, S.P. Reynolds, & B.M. Gaensler
We present recent observations with the Chandra X-ray Observatory of SNR 0453-68.5 in the LMC and SNR 0049-73.6 in the SMC. We have determined that both SNRs resulted from core-collapse explosions. In the case of 0453-68.5, we discovered a pulsar-wind nebula (PWNe) within the central region that pinpointed the origin. This is a large (17-pc radius) SNR, with outer-shell emission well described by a Sedov model. Assuming Sedov dynamics, we arrive at a 13-kyr age for SNR 0453-68.5 and 110 Msun in swept-up material. SNR 0049-73.6 in the SMC is quite a different object. The bright central emission in the central region does not show the presence of a PWNe; rather we find evidence for ejecta. This remnant lacks the limb brightening seen in many remnants as an indication of swept-up ISM material, as the central emission is much brighter than the outer regions. Spectral comparisons of these regions indicate enhanced abundances of O and Ne in the center, consistent with a core collapse SNR. SNR 0049-73.6 has a 24-pc radius, a 15-kyr age, and 140-Msun swept-up ISM. By identifying the bright inner ejecta ring with the reverse shock, we estimate a total ejecta mass of 7.5 Msun in 0049-73.6. The presence of large amounts of O- and Ne-rich ejecta in 0049-73.6 may be contrasted with the lack of detectable ejecta emission in 0453-68.5. We attribute this to a higher initial mass of the SN progenitor in SNR 0049-73.6, adding to growing evidence that explosions of massive progenitors generally do not produce classical Crab- and Vela-like pulsars.
E.M. Schlegel (SAO), A. Kong, P. Kaaret, R. Di Stefano, & S.S. Murray
We describe an observation of the x-ray-luminous SN 1978K in NGC 1313 using the Chandra ACIS detector. Models that provided good fits to ASCA SIS and GIS and ROSAT PSPC spectra no longer do so for the ACIS spectrum. The best-fit model to the ACIS spectrum is a dual hot-plasma model (vmekal); one component is soft (T = 0.61+0.04-0.05 keV, 90% errors) and the other is harder (T = 3.16+0.44-0.40 keV). For the varying abundances permitted within the model, only the Si abundance of the soft component differs from solar, with a value of 3.20+1.80-1.90 (90\% errors). From a ratio of the low- and high-T model fits to the Chandra and XMM-Newton spectra, we infer an exponent of the ejecta density distribution of 5.2 adopting a circumstellar matter distribution exponent of s = 2. The 0.52-keV light curve shows essentially no decline; the 210-keV light curve, constructed only of the ASCA, XMM-Newton, and Chandra observations, shows a drop of 1.5 from the ASCA epoch. Research for this project was supported by contract NAS8-39073 to SAO for the Chandra X-ray Observatory.
J.C. Houck (MIT) & G.E. Allen
SN 1986J ranks as one of the most luminous, x-ray-bright supernovae ever observed. The x-ray emission is attributed to circumstellar interaction with the dense wind from its red-supergiant progenitor star. Because x-ray-bright supernovae are rare and relatively faint, only a small number have been observed in x rays at an age of more than a year or two. We present preliminary results from a recent Chandra observation of SN 1986J which, combined with earlier ASCA and ROSAT PSPC data, suggest that spectral evolution has occurred. In particular, the x-ray absorbing column appears to have increased significantly during the intervening 5 years. Recent radio-spectrum observations by Bietenholz, Bartel, & Rupen (2002) appear consistent with a corresponding increase in radio-frequency absorption.
M. Sasaki (SAO), P.P. Plucinsky, T.J. Gaetz, R.K. Smith, R.J. Edgar, & P.O. Slane
The Galactic supernova remnant (SNR) CTB-109 is thought to be interacting with a molecular cloud complex. As no x-ray emission is observed from the western part of the remnant shell, the cloud complex on this side has apparently stopped the outer blast wave. Inside the shell, CTB-109 has an x-ray bright region in the east, known as the Lobe or the Jet. We present the results from the XMM-Newton AO1 observations of SNR CTB-109. The XMM EPIC spectra of the Lobe shows that its emission is thermal and the spectral variations are probably caused by differences in temperature or in the ionization time scale of plasma. The spectra clearly show Mg and Si lines in the whole Lobe region. The enhanced x-ray emission of the Lobe is indicative of an interaction of the SNR shock wave with a molecular cloud. Furthermore, we derived values for (e.g.) shock velocity or column density from EPIC spectra of the SNR shell. The interaction of the shock with the surrounding medium is discussed. This work was supported by Chandra grant GO0-1127X.
D.A. Leahy (Calgary)
The Cygnus Loop is one of the nearest (440 pc) supernova remnants, allowing studies of spatial variations to be carried out at high physical resolution compared to other supernova remnants. Chandra ACIS observations were obtained covering the bright V-shaped region on the southwest limb. This region has strong 0.22-keV spectral variations seen with the ROSAT PSPC. The nature of the spectral variations is analyzed using the ACIS observations and allowing for spatial variations of elemental abundances. It is found that temperature is the main factor in the spectral variation, but that column density and abundance variations also have a significant contribution.
S. Park (Penn State), J.P. Hughes, D.N. Burrows, J.A. Nousek, G.P. Garmire, & P.O. Slane
0103-72.6 is the second brightest x-ray supernova remnant in the Small Magellanic Cloud (SMC). Our Chandra ACIS observation unambiguously resolves the x-ray emission into a nearly complete, remarkably circular shell surrounding bright bulge emission at the center. The outer shell is dominated by soft x-ray emission, and is the limb-brightened emission from the swept-up SMC interstellar medium. In contrast, the bright central regions are strongly enhanced in the O and Ne abundances, indicating reverse-shock heated ejecta. The detection of O/Ne-enriched ejecta and its location within an HII region attest to a core-collapse origin from a massive progenitor for 0103-72.6.
T. DeLaney (Minnesota), L. Rudnick, R.A. Fesen, U. Hwang, T.W. Jones, R. Petre, & J.A. Morse
We have performed a multi-wavelength comparison of Cassiopeia A using Very Large Array, Hubble Space Telescope, and Chandra X-ray Observatory images. By separating components spectrally, we find clear associations between emission at the three wavebands on scales of 10 to 1. This breaks down at the 1 (0.016 pc) level, indicating that there is not microscopic mixing of the different temperature plasmas. We separate the emitting material into two components shocked circumstellar material (CSM) and shocked ejecta that show the same respective morphologies and proper motions in the different bands. In the shocked CSM, we find matched x-ray low-energy emission and optical QSFs, and x-ray continuum-dominated emission matched with filamentary radio structures. In the ejecta, as defined by x-ray and optical oxygen, silicon, and sulfur line emission and flat-spectrum radio emission, there are large-scale structures likely resulting from the explosion. There is also a great deal of material that is seen only in a single band; these show distinct kinematic structures as well. These different temperature components may represent varying density conditions and/or post-shock evolutionary states.
G.E. Allen (MIT), J.C. Houck, & S.J. Sturner
We present results of a joint spectral analysis of some Chandra ACIS x-ray, MOST radio, and CANGAROO g-ray data for the eastern rim of SN 1006. The x-ray and radio data were fit with a model of synchrotron emission. The g-ray data were fit with a model of Compton scattering of the cosmic-microwave-background radiation. The electron spectrum used for both models is the same as a power-law distribution with an exponential cut off, except that the photon index (G) is a linear function of the logarithm of the electron energy (E): G(E) = G0 + G1 * log(E/[1 GeV]). The results of the analysis provide a self-consistent set of best-fit values (and 90%-confidence intervals) for the spectral index at 1 GeV (G0), the amount of curvature in the electron spectrum (G1), the number density of electrons at 1 GeV, the maximum energy of the electrons, and the magnetic-field strength. The results for the curvature parameter provide strong evidence that the shape of the GeV-to-TeV electron spectrum is not simply a power law, but has some curvature. The results are shown to be qualitatively consistent with theoretical predictions.Back to Topics
L.K. Townsley (Penn State), E.D. Feigelson, T. Montmerle, P.S. Broos, G.P. Garmire, & Y.-H. Chu
We present ACIS images of several high-mass star-forming regions, including the Omega Nebula (M 17), the Rosette Nebula (NGC 2237-2246), and the giant H II region complex W 51. The massive clusters powering these H II regions are resolved at the arcsecond level into hundreds of stellar sources, similar to those seen in closer young stellar clusters. However, we also detect diffuse x-ray emission on parsec scales that is spatially and spectrally distinct from the point-source population. For the nearby regions (M 17 and Rosette) the emission is soft, with plasma temperatures less than 10 MK This is in contrast to what is seen in more distant complexes (e.g., Arches, NGC 3603). This extended emission most likely arises from the fast O-star winds thermalized either by windwind collisions or by a termination shock against the surrounding media. We establish that only a small portion of the wind energy and mass appears in the observed diffuse x-ray plasma. In the blister H II regions, we suspect that most of it flows without cooling into the low-density interstellar medium through blow-outs or fissures in the surrounding neutral material. These data provide compelling observational evidence that strong wind shocks are present in H II regions.
Y.-H. Chu (Illinois), M. Guerrero, & R. Gruendl
Fast (1000-3000 km s-1) stellar winds from hot stars can sweep up the ambient medium into a dense shell, with the interior filled with shocked winds at x-ray emitting temperatures. This schematic picture is applicable to planetary nebulae (PNe), bubbles blown by single massive stars, and super-bubbles blown by OB associations or clusters. The formation and evolution of such bubbles and super-bubbles depend critically on the physical conditions of the interior hot gas, which in turn depend on the clumpiness of the ambient medium, the efficiency of heat conduction between the hot interior gas and the cool shell gas, and the degree of mass loading of cool nebular clumps via wind ablation. The x-ray emission from the hot interiors of PNe, bubbles and super-bubbles is faint, and breakthrough observations have been made only recently by Chandra and XMM-Newton. We review these observations, critically examine the physical conditions of the hot gas in bubble and super-bubble interiors, and discuss their implication on the heat conduction and wind ablation at the interfaces between the hot interiors and the cool shells. Chandra observation of NGC 6888 and XMM-Newton observation of S308 are highlighted.
A. Juett (MIT), N.S. Schulz, & D. Chakrabarty
We have used photoelectric absorption features in Chandra HETGS spectra of seven bright x-ray binaries, to study the detailed spectroscopic structure of oxygen absorption in the interstellar medium (ISM). This represents the highest-resolution x-ray spectral study of interstellar O absorption ever performed, revealing previously undetected features and demonstrating the inadequacy of existing models for grating data. In particular, we find that the O edge structure is well fit by a model that includes 5 absorption lines and 2 edges. We also identify the absorption lines at 23.51 and 22.88 �as the 1s2p and 1s3p resonance lines of neutral O. The best-fit wavelength of the stronger edge is consistent with the spin-3/2 resonance series in neutral O, while the spin-1/2 resonance series is less firmly detected. The remaining features are attributed to the 1s-2p lines of singly and doubly ionized O and allow an estimate of the relative abundance of ionized O in the ISM. This work is part of a larger program to produce a reliable high-resolution spectral model for interstellar absorption.
E. Costantini (MPE) & P. Predehl
Interstellar dust scatters X radiation coming from background objects producing the effect of a halo of diffuse and faint emission around the point source. The new generation x-ray telescopes led to a deeper understanding of this process. The scattered radiation energy distribution could be studied for the first time and more accurate dust models could be tested. Here we present the analysis of 10 Chandra public data sets of strong x-ray emitters in the Milky Way, observed by ACIS-I and ACIS-S. The brightest haloes occur for high ( 1022 cm-2) column densities; therefore, in this study we focus on sources with galactic latitude b < 3 in order to have a halo contribution >10% (Predehl & Schmitt 1995). The modeling of the extended emission, which provides information of the dust size distribution and the distribution of the matter along the line of sight, is discussed. Chandras point spread function (PSF) allowed us to study the extended emission down to few arcseconds, where the contribution of large dust grains or grains concentrated close to the source play a role. On the other hand, the spectrum of the scattered radiation, provided that very good statistics were available, gave information on the chemistry of the scattering particles. We also compare the present results with XMM-Newton findings (Costantini et al. 2003) pointing out the complementarity of the two telescopes in the study of interstellar dust.
R.J. Edgar (SAO), R.K. Smith, P.E. Freeman, P.P. Plucinsky, & B. Biller
We present results of a 100-ks Chandra observation (obsid 943) of MBM 12, a nearby [d 90 pc, (l, b) = (159.1, -34.5)] molecular cloud. Snowden, McCammon, & Verter (1993) used deep ROSAT PSPC observations of this cloud to measure the foreground 0.25-keV band emission from the Local Bubble. They also put strong upper limits on the foreground 0.51.0 keV (M-band) emission. Most, if not all, models for the Local Bubble predict that the emission in this band is primarily due to O VII and O VIII lines. We report detection with high statistical confidence of oxygen emission arising between the observer and this cloud. Many statistical and systematic effects are considered including low-energy ACIS quantum efficiency and redistribution functions, charged-particle backgrounds, and statistical biases. We acknowledge support from NASA contract NAS8-39073 with the Chandra X-ray Center, and Chandra guest observer grant GO0-1097X.Back to Topics
G.G. Pavlov (Penn State), O. Kargaltsev, & D. Sanwal
Chandra and XMM-Newton observations of about 50 isolated (non-accreting) pulsars have provided valuable data on the x-ray properties of these objects. Due to the higher sensitivity and better spectral resolution of the Chandra ACIS and XMM EPIC detectors, we can separate thermal and nonthermal components of their spectra and study radiation from the neutron-star surfaces and magnetospheres with much higher certainty than possible in the pre-Chandra era. Particularly important is the high spatial resolution of Chandra data, which allows one to separate the pulsar radiation from that of compact pulsar-wind nebulae (PWNe) around young pulsars and to study the PWNe morphology and spectra. Monitoring observations of the brightest PWNe (around the Crab and Vela pulsars) revealed stunning pictures of their complicated dynamic behavior. The large sample of pulsars and PWNe observed with the Chandra and XMM-Newton observatories make it possible to examine correlations between various properties of these objects and their evolution. We present an overview the general x-ray properties of pulsars and PWNe and present the most interesting results of observations of individual objects. This work is partly supported by SAO grants GO2-3089X, GO2-3091X, GO3-4091X and NASA grant NAG5-10865.
C. Kouveliotou (MSFC/NSSTC)
Magnetars are a subclass of highly magnetized neutron stars characterized by their recurrent x-ray bursts. While in an active (bursting) state, the sources emit hundreds of predominantly soft (kT 30 keV), short (0.1100-ms) events. Active states last from days to years. Their quiescent-source x-ray light curves exhibit pulsations in the narrow range of 58 s; estimates of these rotational period rate changes (spin-down) indicate that their magnetic fields are extremely high, of the order of 10141015 G. Such high-magnetic-field objects, dubbed magnetars, had been predicted in 1992; however, the first concrete observational evidence was obtained in 1998 for two of these sources. I discuss here the history of magnetars and their x-ray spectral, timing, and flux characteristics, both in their persistent and burst emission. Finally, I describe the prevailing theories in the field.
D. Sanwal (Penn State) G.G. Pavlov, V.E. Zavlin, & K. Moody
1E 1207.4-5209, the compact central object in SNR PKS 1209-51/52, is an intriguing radio-quiet x-ray pulsar. It has a 424-ms period, discovered with Chandra. Chandra observations also found absorption lines in its spectrum, the first spectral lines detected from an apparently isolated neutron star. X-ray timing observations with Chandra and XMM-Newton show peculiar behavior in the spin properties of this source. Deviation from a uniform spin-down suggests that the pulsar either glitches or is in a wide detached binary with a low-mass companion. Studying the spin properties of this source is extremely important to understanding its nature, including the origin of the spectral lines. Recent optical/IR observations with VLT and HST have found a very red candidate counterpart. We present the latest x-ray timing results and the optical/IR observations and discuss possible models of this important and peculiar source.
X. Zhang (UAH), R.X. Xu, & S.N. Zhang
Featureless spectra of isolated neutron stars may indicate that they are actually bare strange stars (BSS), but a definitive conclusion on the nature of these compact objects cannot be reached until accurate, theoretically calculated spectra of the bare quark surface are known. However, due to the complex nonlinearity of quantum chromodynamics, it is almost impossible to produce a definitive and accurate calculation of the density-dominated quarkgluon plasma from first principles. Nevertheless, it was suggested that cold quark matter with extremely high baryon density could be in a solid state. Within the realms of this possibility, we fit the 500-ks Chandra LETGS data for the brightest isolated neutron star RX J1856.5-3754 with a phenomenological spectral model, and find that electric conductivity of quark matter on the stellar surface is about > 0.181018 s-1.
D.A. Leahy (Calgary)
An accretion-column model that includes relativistic light bending effects and allows various emissivities is applied to model the pulse shape of Her X-1. The initial model is motivated by the observed 35-day cycle of pulse-shape changes (Scott, Leahy, & Wilson 2000, ApJ, 539, 392). The observed pulse-shape changes imply a pencil beam from the near pole and a fan beam from the far pole. The results from applying the accretion-column model are given. Fairly strong constraints on the geometry of the accretion column and on the radius of the column emissivity result, as do weaker constraints on the neutron-star radius.
S.K. Patel (NRC/NSSTC), C. Kouveliotou, A.F. Tennant, P.M. Woods, A. King, P. Ubertini,, C. Winkler, T.J.-L. Courvoisier, M. van der Klis, S. Wachter, B.M. Gaensler, & C.J. Phillips
The new transient IGR 16358-4726 was discovered on 2003 March 19 with INTEGRAL. We detected the source serendipitously during our 2003 March 24 observation of SGR 1627-41 with Chandra at the 1.710-10 ergs s-1 cm-2 flux level (210 keV) with a very high absorption column (NH = 3.31023 cm-2) and a hard power-law spectrum of index 0.50.1. We discovered very strong flux modulation with a period of (588050) s and peak-to-peak pulse fraction of (706)% (210 keV), clearly visible in the x-ray data. We observed IGR 16358-4726 again on 2003 April 21 using Chandra and found that the unabsorbed flux has decreased by a factor of about 15; we clearly detected the 1.62-hr modulation again, as well as significant flickering in the sources light curve. To date, the nature of IGR 16358-4726 remains unresolved. The only neutron-star systems known with similar spin periods are low-luminosity persistent wind-fed pulsars. If the 1.62-hr modulation is a spin period, this transient is a new kind of object; if it is an orbital period, then the system could be a compact Low-Mass x-ray Binary (LMXB).Back to Topics
J.E. Grindlay (Harvard/CfA)
The remarkable angular resolution of Chandra has made possible a broad range of new studies of globular clusters from rich details of the previously suspected quiescent LMXB and CV populations, to discoveries of the significant populations of millisecond pulsars and chromospherically active main-sequence binaries in galactic globular clusters, and to wholly new studies of globular-cluster systems in external galaxies. I provide a broad overview of many of the major discoveries and then focus on our own deep survey of perhaps the most remarkable globular yet studied with Chandra 47 Tuc. New results for the total source population, luminosity functions, variability, and spectra of these sources are presented and compared with other well-studied globular clusters (e.g., NGC 6397). I am indebted to many colleagues in this ongoing work particularly Craig Heinke and Peter Edmonds and to various Chandra grants for support.
C. Heinke (SAO), J.E. Grindlay, & P.D. Edmonds
Recent Chandra x-ray observations of globular clusters have identified numerous accreting neutron stars in quiescence, with spectra generally well described by hydrogen-atmosphere models. Spectral analysis of these systems at known distances allows constraints upon the neutron-star mass and radius. These constraints are similar but not identical to the R contours appropriate for blackbody spectra. The quiescent NS X7 in 47 Tuc appears to require a mass greater than 2 Msun if its radius is between 9 and 16 km, implying a stiff neutron-star equation of state.
W.E. Becker (MPE), M.C. Weisskopf, D.A. Swartz, G.G. Pavlov, R.F. Elsner, J.E Grindlay, R. Mignani, A.F. Tennant, D. Backer, L. Pulone, & V. Testa
We report results of the first Chandra observations of the globular cluster M28 (NGC 6626). The observations detected 46 x-ray sources in M28, of which 12 lie within one core radius of the center. We show that the apparently extended x-ray core emission seen with the ROSAT HRI is due to superposition of multiple discrete sources, for which we determine the x-ray luminosity function down to 61030 erg s-1. The data allow the first measurement of the radial distribution of x-ray sources in M28. A fit to a King profile yields a core radius of about 11. One of the main goals of the observation was to measure the unconfused phase-averaged x-ray spectrum of the 3.05-ms pulsar B1821-24. We find that the pulsar spectrum is best described by a power-law photon index 1.2, with marginal evidence for an emission line centered at 3.3 keV in the pulsar spectrum. Interpreting the line feature in terms of cyclotron emission from a corona above the pulsar's polar cap would suggest a magnetic field is very different from a centered dipole. The unabsorbed pulsar flux in the 0.58.0-keV band is 3.510-13 ergs s-1 cm-2. In addition to the pulsar spectrum, we present spectral analysis of the 5 brightest unidentified x-ray sources in M28. Using the Chandra-derived positions, we also report on our search of archival Hubble Space Telescope data for possible optical counterparts of the x-ray sources in M28.Back to Topics
S. Komossa (MPE)
In the first part of the talk, observational evidence for the existence of supermassive black-hole binaries is summarized, with emphasis on recent x-ray observations of ultra-luminous infrared galaxies. The prospects for detecting more binary black holes with the Chandra observatory and with future generations of x-ray satellites are outlined. The second part summarizes and provides new evidence for stellar tidal capture and disruption events, seen in x rays as large amplitude flares. These different observations allow probing the properties and evolution of black holes, and provide important constraints on galaxy evolution scenarios.
J.M. Miller (CfA)
The advanced spectrometers aboard Chandra and XMM-Newton are well-suited to the high fluxes observed from stellar-mass Galactic black holes, and have clearly revealed relativistic iron emission lines in these systems. The observed line profiles are strikingly similar to those first clearly revealed in ASCA observations of some Seyfert galaxies. These relativistic lines can be used to constrain the angular momentum of the black hole and the nature of the accretion flow, and to study connections between stellar-mass and supermassive black holes. I discuss these topics, review recent results, and offer some commentary on how future observations with Chandra and XMM-Newton can exploit broad iron lines to resolve some outstanding issues in the study of Galactic black holes.
S. Migliari (Astron. Inst. Anton Pannekoek), R. Fender, & K. Blundell
In 2000 we made a 10-ks Chandra ACIS-S observation of the archetypal Galactic jet source SS 433 showing arcsecond-scale x-ray jets whose spectra revealed a hot continuum and Doppler-shifted iron emission lines. This was the first direct evidence for ongoing (re-)heating of thermal plasma downstream in a jet from an x-ray binary system. We now present a 60-ks Chandra ACIS-S observation of the jets of SS433 simultaneous with a 40-ks Very Large Array (VLA) radio observation. Spatially resolved x-ray spectra reveal multiple, possibly double-peaked, emission lines from the arcsecond-scale x-ray jets. Comparison with the radio image demonstrates that the thermal and relativistic synchrotron-emitting plasmas are physically coincident. Comparison with previous Chandra imaging furthermore demonstrates that the arcsecond-scale x-ray jets are not static features formed over many precession cycles but are varying significantly, probably continuously, as the jet precesses. Finally, the Chandra images indicate that there may be a strong, thermal, outflow perpendicular to the jets, probably in the equatorial plane of the system.
M.R. Garcia (SAO), J.M. Miller, J. McClintock, A. King, & J.A. Orosz
We point out that the four spatially resolved relativistic jets among the 14 dynamically confirmed black-hole x-ray novae are all in systems with long orbital periods. Many shorter period systems show transient radio outbursts that are attributed to jets, but these jets have not been spatially resolved. Super-Eddington accretion has been suggested as a requirement for jet formation and may be consistent with our compilation of luminosities; however, some super-Eddington outbursts did not form spatially resolved jets. We speculate that some as-yet-unknown process (or combination of processes) favors formation of substantially larger jets in long period systems. Two short-period systems show evidence for extent, but have not been resolved into multiple components, as have the long-period systems. We acknowledge support of CXC contract NAS8-39073 and LTSA grant NAGW-10889.
Y. Yao (UAH), S.N. Zhang, X. Zhang, & Y. Feng
With excellent angular resolution, good energy resolution, and broad energy band, the Chandra ACIS is the best instrument for studying the x-ray halos around some Galactic x-ray point sources, due to scattering by interstellar dust. However, direct images of bright sources obtained with ACIS usually suffer from severe pile-up. Making use of the fact that an isotropic image could be reconstructed from its projection into any direction, we can reconstruct the images of the x-ray halos from data obtained with the HETGS and/or in continuous clocking (CC) mode. These data have no (or less serious) pile-up and enable us to take full advantage of the excellent angular resolution of Chandra. With the reconstructed high-resolution images, we can probe x-ray halos as close as 1 to the associated point sources. Applying this method to Cygnus X-1 observed with Chandra HETGS in CC mode, we derived an energy-dependent radial halo flux distribution. From this, we conclude the following: (1) In a circular (2-radius) region centered at the point source, the fractional halo intensity is about 15% near 1 keV and drops to about 5% at about 6 keV; (2) about 50% of the halo photons fall within a 40-radius region; and (3) the spectrum of the point source is slightly distorted by the halo contamination.
G.A. Richardson (NRC/NSSTC) & K.-I. Nishikawa
With observations from the Chandra X-ray Observatory, our understanding of the formation and propagation of relativistic jets is improving. We present our numerical simulation method for solving the fluid equations in general relativistic environments with magnetic fields. Our motivation for developing such a method is to study the environment around a rotating black hole, specifically the dynamics of the accretion disk and the associated formation of relativistic jets. We present and compare our 2-D and 3-D simulation results, which demonstrate the initial stages of jet formation.
J.A. Tomsick (UCSD), S. Corbel, R. Fender, J.M. Miller, J.A. Orosz, M.P. Rupen, T. Tzioumis, R. Wijnands, & P. Kaaret
Using Chandra, we have detected the black-hole transients V4641 Sgr and XTE J1859+226 in their low-luminosity, quiescent states. The 0.38 keV luminosities are (4.0+3.9-2.1)1031 (d/[7 kpc])2 erg s-1 and (4.2+4.8-2.2)1031 (d/[11 kpc])2 erg s-1 for V4641 Sgr and XTE J1859+226, respectively. With the addition of these 2 systems, 14 of the 15 transients with confirmed black holes (via compact-object mass measurements) now have measured quiescent luminosities or sensitive upper limits. The only exception is GRS 1915+105, which has not been in quiescence since its discovery in 1992. The luminosities for V4641 Sgr and XTE J1859+226 are consistent with the median luminosity 21031 (d/[11 kpc])2 erg s-1 of previously detected systems. Our analysis suggests that the quiescent x-ray spectrum of V4641 Sgr is harder than for the other systems in this group, but uncertainties are large due to limited statistics. Focusing on V4641 Sgr, we compare our results to theoretical models for x-ray emission from black holes in quiescence. Also, we obtain precise x-ray positions for V4641 Sgr and XTE J1859+226, via cross-correlation of the x-ray sources detected near our targets with IR sources in the 2-Micron All-Sky Survey catalog.
J.A. Tomsick (UCSD), E. Kalemci, & P. Kaaret
In order to constrain theoretical models for x-ray emission from black holes in the low-hard state and (potentially) quiescence, we studied the black-hole candidate XTE J1650-500 with Chandra and RXTE near the end of its 20012002 outburst after its transition to the low-hard state at x-ray luminosities down to L = 1.51034 erg s-1 (d/[4 kpc])2. Our results include a characterization of the spectral and timing properties. At the lowest sampled luminosity, we used an 18-ks Chandra observation to measure the power spectrum at low frequencies. The 0.520 keV energy spectra are consistent with a power-law model with interstellar absorption, and the source softens at the lowest luminosity. The power spectra are characterized by strong (20-35% fractional rms) band-limited noise, which we model as a zero-centered Lorentzian. From the soft-to-hard state transition (at L = 71036 erg s-1) to our lowest luminosity observation, the Lorentzian half-width drops by a factor of nearly 1200. If this frequency scales with the Keplerian frequency at the inner edge of the accretion disk, this implies a change in the disk inner radius by a factor of 110. This would indicate that large inner radii can occur (as predicted by ADAF and sphere+disk models), but that extremely large radii are not required for the source to show hard-state properties. We also discuss possible implications for models where the low-hard-state emission is produced in a jet, as well as the question of whether quiescence is a low-luminosity version of the hard state.
Y. Feng (UAH/NSSTC), S.N. Zhang, A.F. Tennant, X.L. Zhang, & Y.S. Yao
Spectral features around 58 keV have been detected in GRS 1915+105 with ASCA, BeppoSAX, RXTE, and Chandra. In some observations, a broad emission-line feature was present, with an equivalent width varying over 70700 eV. The profile of the broad emission-line feature was also found to change, implying that the configuration of accretion flow (disk, corona, and outflow) changed in some observations. We analyzed the observations covering high and low radio flux states and low/hard and high/soft x-ray spectral states. Timing properties and changes in continuum spectrum also suggest variations in the accretion flow. We report the variation of the broad-line feature and compare with continuum spectrum and timing properties. The possible origin of the broad emission-line feature and the physical implication of our results are discussed.
S. Dieters (Georgia Southwestern State), R. Sood, & P. O'Neill
Using the RXTE ASM and BATSE earth occultation x-ray flux histories of GX1+4, we have conducted a thorough search for periodic modulations. We used two approaches a Fourier-based search for broad signals and a Z-n test search for the regular recurrence of short duration flares that are evident in the light curve of GX 1+4. Both approaches yield no evidence for a periodicity in the flux of GX 1+4 at previously reported periods or over the 202000 day search interval. We have set upper limits on both broad and low duty-cycle signals.
E. Kellogg (SAO), J. Nichols, J. Pedelty, & J. Sokoloski
We review observations and models of x-ray jets observed in compact stars in our Galaxy, focusing primarily on white dwarfs. We summarize observations to date, especially with Chandras high-resolution imagery and spectral capabilities, and resulting advances in understanding jet phenomena. Our prototype is the nonrelativistic jet in the symbiotic star R Aqr. We discuss x-ray and radio images and x-ray spectra, with comparisons to observations at other wavelengths including IUE, VLA, HST, and ground-based optical observatories. We compare with observed micro-quasar jets, relativistic jets from neutron stars and from black-hole candidates, as well as collimated outflows in planetary nebulae. We discuss prospects for observing jets from other symbiotic and related systems. We describe mechanisms for jet formation including shocks, magnetic collimation, with the important underlying influence of rotation of the white dwarf, powered by accretion. We also discuss time dependence of the jet, both that observed from the few observations and that expected based on the outcome of x-ray spectral modeling.
B.F. Williams (CfA), M.R. Garcia, A.K.H. Kong, F.A. Primini, & S.S. Murray
We are monitoring the x-ray point-source population in M31, discovering and analyzing dozens of transient x-ray sources likely to be siblings of Galactic low-mass x-ray binary (LMXB) transients. We have combined Chandra and XMM-Newton data to study the long-term light curves of these sources. The location of the majority of these sources in the M31 bulge implies that they have low mass. The x-ray light curves are not unlike those of Galactic black-hole x-ray novae. These two facts suggest that these are indeed black-hole transients in M31. We discuss the light curves and physical parameters for such transient x-ray sources in M31. If our sample is representative of the population of black-hole binaries in M31, the number of these sources is surprisingly large. Indeed, we are finding roughly as many black-hole primaries as neutron-star primaries in LMXBs in the M31 bulge. Support for this work was provided through NASA contract NAS8-39073 to the Chandra X-ray Center and through Chandra grant number GO2-3103X.
P. Jonker (IoA), E. Gallo, V. Dhawan, M. Rupen, R. Fender, & G. Dubus
We present the results of our recent Chandra and VLA DDT observations of the Galactic black-hole candidate (BHC) XTE J1908+094 during its decay towards quiescence. The main goal of these observations was to test the whether the observed radiox-ray correlation found by Gallo et al. (2003) and Corbel et al. (2003) in two other Galactic BHCs over 34 decades in flux persists all the way towards quiescence. If this were the case, this would validate one of the assumptions made by Fender, Gallo, & Jonker (2003). Previous radio and x-ray observations of the BHC A0620-00 in quiescence also suggest that the radiox-ray relation can be extended towards quiescence. We show that, due to the relative faintness of these BHC systems in radio in quiescence, one can observe the radiox-ray correlation best in a nearby BHC in order to be able to follow the source all the way towards quiescence.Back to Topics
L. Piro (IASF/Rome)
X-ray observations of gamma-ray bursts (GRBs) have provided the key to measuring their distance scale, unveiling also a new aspect of this astrophysical phenomenon namely, the afterglow emission. We review the main properties of the afterglow, with particular attention to the x-ray afterglow, its link with the main pulse in the framework of the fireball scenario, the connection of GRB with supernova-like explosions from massive stars, and new classes of dark and x-ray rich GRBs. Finally, we briefly discuss GRBs as cosmological beacons from the early to local universe.
N. Schartel (ESA/Vilspa)
The unprecedented large effective area of the XMM-Newton EPIC cameras, in combination with simultaneous optical/UV imaging (OM) and x-ray spectroscopy (RGS) capabilities, provides a unique opportunity to study the spectra of gamma-ray bursts (GRBs). First I summarize briefly the technical/operational constraints and challenges of GRB observations with XMM-Newton. Then I main provide an overview of GRB observations performed by XMM-Newton focusing on the scientific results.
G.R. Ricker (MIT) & HETE Science Team
The HETE satellite has observed more than 250 gamma-ray bursts (GRBs) in 2.5 years of operations. It is currently localizing 2530 GRBs per year, having localized 43 GRBs thus far. Twenty-one of these localizations have led to the detection of x-ray, optical, or radio afterglows; 11 of the bursts with afterglows have redshift determinations. Rapid x-ray, optical, IR, and radio follow-up identifications of HETE GRBs over the past year have revealed the nature of dark bursts and x-ray flashes (XRFs). Follow-up observations of HETE bursts with Chandra imaging and spectroscopy have proven to be extremely productive in relating GRBs to their progenitors, to their local ISM, and to their host galaxies. We discuss the implications of 12 Chandra observations of 8 HETE-discovered GRBs GRBs 011130, 020531, 020813, 021004, 030226, 030328, 030528, and 030723. The HETE Science Team includes participants from France, Japan, Brazil, India, Italy, and the USA. This research was supported in the USA by NASA contracts NASW-4690 and NAS8-39073.Back to Topics
F.K. Baganoff (MIT), M.W. Bautz, G.R. Ricker, W.N. Brandt, G. Chartas, E.D. Feigelson, G.P. Garmire, M. Morris, E.E. Becklin, A.M. Ghez, S.D. Hornstein, A.M. Tanner, A.S. Cotera, P.M. Hinz, W.F. Hoffmann, M.R. Meyer, A. Eckart, R. Genzel, J.-H. Zhao, R.M. Herrnstein, J.L. Hora, J.-P. Macquart, Y. Maeda, R.J. Sault, & G.B. Taylor, F. Walter
The supermassive black hole in the Galactic center, Sgr A*, has been observed repeatedly with the ACIS-I instrument on the Chandra X-ray Observatory. The bulk of the data come from a 0.5-Ms exposure during the period 2002 May 22 to June 4. This extended observation detected 3 x-ray flares with amplitudes exceeding a factor of 10 and several flares exceeding a factor of 5. Flare durations ranged from 0.5 to several hours with rise and fall times of a few hundred seconds, similar to the original x-ray flare discovered with Chandra in 2000. Two additional flares have been detected with XMM-Newton. These observations confirm that rapid x-ray flaring of Sgr A* is common. Averaging over all Chandra observations from 19992002, a total of 0.6 Ms, the mean rate of factor-of-10 flares is 0.60.3 per day. A network of ground-based telescopes were employed for simultaneous observations Keck, VLT, and Magellan in the IR; SMA, ATCA, OVRO, and VLBA in the mm; and VLA in the cm. During the x-ray flares, no significant variations were detected at the longer wavelengths. There may be some evidence for mm variations at a few tens of percent level; the current upper limits are about 50%. The quiescent x-ray emission within 1.5 of Sgr A* shows a strong emission line from highly ionized iron and is resolved by Chandra on a scale consistent with the Bondi capture radius for the black hole. This work was supported by Chandra Award G02-3115A issued under NASA Contract NAS8-39073.
F. Yusef-Zadeh (Northwestern), M.P. Muno, M. Wardle, F.K. Baganoff, M. Pound, W. Cotton, N. Kassim, M. Nord, J. Lazio, M. Morris, D. Pierce-Price, C. Law, G.P. Garmire, & S. Park
Recent Chandra and XMM-Newton observations reported evidence of a nonthermal x-ray filament in the Galactic center (Sakano et al. 2003, MNRAS, astro-ph 0211572). The x-ray source XMM J174540-2904.5 has a hard spectrum with a 15 extent and coincides with a wisp-like synchrotron radio source called SgrA-E (Ho et al. 1985, ApJ, 288, 275). Here, we present more sensitive VLA and Chandra, as well as new BIMA observations, of XMM J174540-2904.5 and report the detection of an additional wisp (SgrA-F) of nonthermal x-ray emission. We discuss whether the x-ray emission from the SgrA-E and F sources are due synchrotron or due to the Compton scattering of the FIR emission from the adjacent 20 km s-1 molecular cloud. We also show that sources E and F coincide with the brightest portion of a shell-like radio feature, which is likely to be a supernova remnant.Back to Topics
S.W. Allen (IoA)
Chandra observations of rich, dynamically relaxed galaxy clusters allow precise determination of properties of the x-ray gas and the total gravitating mass. The measured mass profiles, which are confirmed by independent gravitational-lensing studies, exhibit a form in good agreement with the predictions from numerical simulations. Combining Chandra results on the x-ray gas mass fractions in clusters with independent measurements of the Hubble parameter and the mean baryonic-matter density of the universe, we obtain a tight constraint on the mean total matter density of the universe Wm, and an interesting constraint on the dark-energy density WL. Using these results together with the observed local x-ray luminosity function of the most x-ray-luminous galaxy clusters, a mass-luminosity relation determined from Chandra and ROSAT x-ray data and weak gravitational lensing observations, and the mass function predicted by numerical simulations, we obtain a precise constraint on the normalization of mass fluctuations in the universe s8. I discuss the remarkably tight constraints on cosmological parameters obtainable from the combination of current x-ray-cluster and cosmic-microwave-background data. These results include a preference for a non-zero neutrino mass.
C. Scharf (Columbia), I. Smail, R. Ivison, R. Bower, W. van Breugel, & M. Reuland
The powerful radio galaxy 4C 41.17 resides in a proto-cluster environment at z = 3.8, consisting of an overdensity of sub-mm, Lyman-break, and Lyman-emission objects. Using Chandra we detected a 100-kpc extended x-ray structure in this system. It appears to be inverse-Compton emission from a combination of the enhanced CMB at this redshift and the high luminosity far-IR photons associated with the vigorous star formation seen in the sub-mm. We compare the emission to a giant Ly-a emission halo seen around 4C 41.17 and propose that we are seeing the equivalent of the low-z intra-cluster cavities, blown by radio jets. At high z however the Compton emission from these structures can significantly contribute to photo-ionization of infalling gas and may provide a new physical feedback mechanism relevant to both the formation of clusters and the truncation of massive-galaxy growth.
E.L. Blanton (Virginia), C.L. Sarazin, B.R. McNamara, & T.E. Clarke
Recent observations of clusters of galaxies with Chandra have revealed the profound effect that central radio sources have on the intra-cluster medium (ICM). The radio sources inflate bubbles in the ICM that then rise buoyantly transporting energy and magnetic fields into the cluster. The energy transferred to the ICM from several outbursts of the radio sources is a leading candidate for the solution to the cooling-flow problem (sufficient quantities of gas are not seen at sufficiently low temperatures), in that the radio sources can heat the gas cooling in the centers of clusters. Results of the analysis of Chandra data of cooling-flow clusters with central radio sources are presented, with emphasis placed on the energy transferred to the ICM from the central objects in Abell 262 and Abell 2052.
M. Bonamente (UAH), M.K. Joy, J. Carlstrom, & E. Reese
We study Chandra x-ray data and SunyaevZeldovich-effect (SZE) data in a large sample of galaxy clusters of moderate-to-high redshift (z = 0.160.89) to determine the cosmic scale distance. The Chandra x-ray data and BIMA and OVRO interferometric radio data are combined to measure cluster distances independently of the extragalactic distance ladder. We employ a Monte-Carlo Markov-chain (MCMC) for the simultaneous determination of best-fit values and confidence intervals of all the model parameters. This method is computationally more efficient than likelihood-ratio methods; it is here applied for the first time to the x-raySZE data of galaxy clusters.
M.W. Bautz (MIT), J. Arabadjis, T. Jeltema, E. Ellingson, & G.P. Garmire
The Red-sequence Cluster Survey (RCS) of Gladders & Yee (2000) exploited the colormagnitude relationship of cluster galaxies to identify high-redshift galaxy clusters in an approximately 100-square-degree survey area. We report results of Chandra observations of several of the highest-redshift (z > 0.8) objects detected in the RCS. We find that the x-ray luminosities are lower, by factors of several or more, than expected on the basis of the observed optical richness. Where available, the plasma temperature in these RCS clusters is also significantly lower than expected. We discuss implications of these results for the dynamical states of these clusters.
A.A. Vikhlinin (SAO), M. Markevitch, P. Mazzotta, S.S. Murray, W.R. Forman, C. Jones, & R.P. Kraft
In a number of galaxy clusters, Chandra has observed edges in the surface brightness that were found to correspond to the contact discontinuities between the dense cold gas and the hotter ambient medium. These structures were named cold fronts. We review observations of cold fronts and merger shocks in several clusters and present their applications to the physics of the intra-cluster medium, such as large-scale gas motions, magnetic fields, and turbulence.
B.R. McNamara (Ohio)
One of the surprising results that has emerged from Chandra and XMM-Newton observations of galaxy clusters is the discovery that cooling flows are dramatically smaller than previously thought. I show that new x-ray limits on cooling are nearly in accordance with star-formation rates derived from optical and ultraviolet measurements of many cD galaxies. In addition, the sites of star formation and bright emission nebulae are associated with what appear to be cooling condensations in the x-ray gas. I discuss energy feedback into the intra-cluster medium by supernova explosions associated with recent star formation, heat conduction from the hot outer layers of clusters, and x-ray cavities created by interactions between powerful radio sources and the intra-cluster medium. I show that buoyant cavities in some systems contain enough energy to reduce significantly or quench cooling.
E. De Filippis (MIT) & M.W. Bautz
We present new results on the galaxy clusters Abell S0177 (AC 114), which, with its strong and uncommon dynamical activity, is providing an incredibly rich ground for studying phenomena related to single and multiple merger events. In the past years, weak and strong lensing analyses have estimated the mass distribution of the cluster as composed of an additional bimodal component, formed by two clumps located on opposite sides of the cluster core. Our new x-ray analysis gives partial confirmation of the lensing results, with the detection of an excess of x-ray emission at the position of the Southern clump. We also report the discovery of a soft x-ray filament connecting the cluster core to the clump. This suggests a possible recent passage, close to the cluster core, of a sub-structure that gets stripped of its intra-group cool gas leaving a soft tail behind it. The Northern part of the cluster reveals signs of further strong dynamical activity. Two sharp discontinuities in both the surface brightness and temperature profiles are observed, pointing to another recent merger event which occurred in or close to the cluster core, with one of the two merging substructures moving towards North-East creating a sharp edge in front and a more diffuse feature behind. The x-ray analysis gives us new fundamental clues on the matter distribution of this cluster. We are using the x-ray information as further constraints for a new estimate of the total cluster mass and of its distribution via strong lensing.
M.W. Wise (MIT) & J.C. Houck
Chandra and XMM-Newton data of the cores of clusters have dramatically changed our view of the traditional cooling-flow picture. In particular, the lack of strong emission lines from gas at temperatures below 3 keV in XMM-Newton Reflection-Grating Spectrometer (RGS) and Chandra High-Energy Transmission-Grating Spectrometer (HETGS) spectra has become a key argument for cooling-flow models that include such additional physics as AGN heating, thermal conduction, star formation, and internal absorption. Due to observational limitations, RGS and HETGS spectroscopy is, however, typically limited to studying the cooling-flow spectrum from the cluster as a whole. Alternatively, Chandra ACIS imaging of clusters can sample the cluster temperature structure in the core directly albeit at reduced spectral resolution. We present the results from a spatially resolved, spectral analysis for a sample of cooling and non-cooling flow clusters. Using a simple, parameterized differential-emission-measure model, we determined the departures from the canonical, isobaric cooling-flow model for each cluster, both globally and as a function of position in the cluster. We compare these results to recent XMM-Newton RGS spectra and expectations from modified cooling-flow models that include heating and absorption.
C. Scharf (Columbia), D. Zurek, & M. Bureau
We present first results of the Chandra Fornax survey, a 0.5-Ms total exposure covering the inner 1 of the Fornax cluster of galaxies. At a distance of 20 Mpc, Chandra can resolve structure in Fornax as small as 0.1 kpc sufficient to identify integrated x-ray emission from individual globular clusters in member galaxies and to probe the temperature and abundance structure of the intra-cluster medium in unprecedented detail. We present images, temperature maps, and identifications with a range of intra-cluster populations (e.g., dwarf galaxies) and discuss implications for our picture of cluster properties and formation histories.
K. Hayashida (Osaka), T. Shiroshoji, Y. Nishino, & H. Katayama
A1674 is a richness-class-3 cluster at z = 0.106. Despite its richness, the x-ray luminosity is as low as 51043 erg s-1. ASCA observations of this cluster revealed extended x-ray emission elongated toward NE-SW direction. The ASCA x-ray spectrum of this cluster was fitted with a transparent thermal-plasma model of 3-keV gas temperature and metal abundance 0.0 (90% upper limit of 0.2). Its low metal abundance is strange considering the large number of galaxies and its low x-ray luminosity, since the metal must be supplied from the galaxies to the hot gas (Hashimotodani et al. 2000, Adv. Space Res., 2, 611). The cluster was observed with XMM-Newton. Here we present the results from the XMM-Newton observation, focusing on the metal abundance determined from the high quality EPIC x-ray spectra.
H. Katayama (Tsukuba) & K. Hayashida
We study the total gravitating mass distribution in the central region of 23 clusters of galaxies with Chandra. Using a new de-projection technique, we measure the temperature and gas density in the very central region of the clusters as a function of radius without assuming any particular models. Under the assumptions of hydrostatic equilibrium and spherical symmetry, we obtain the de-projected mass profiles of these clusters. The mass profiles are nicely scalable with a characteristic radius (r200) and mass (M200) on the large scale of r > 0.1 r200. In contrast, the central (r < 0.1 r200) mass profiles have a large scatter even after the scaling. The core radii of the total mass density profiles are systematically larger than those of the gas density profiles, suggesting that gas is more concentrated than dark matter. The inner slope a of the total mass density profile (r(r) ra) is derived from the slope of the integrated mass profile. The values of the inner slope a at the radius of 0.02 r200 (a0) span a wide range from 0 to 2.3. For 6 of 20 clusters, a0 is lower than unity at 90% confidence. Cold Dark Matter (CDM) simulations predict that the inner slope 1 < a < 2, inconsistent with our results. We compare our results with some theoretical models of dark matter.
J.S. Arabadjis (MIT) & M.W. Bautz
We present a spectral de-projection analysis of a sample of dynamically relaxed galaxy clusters. We test each cluster for the presence of multiphase gas and derive a temperature profile. We compare the resulting gravitating mass profiles to those derived from gravitational lensing experiments and to the results of numerical experiments. We find that the data are generally consistent with weak lensing measurements and cold dark matter (CDM) simulations. In some cases, however, the cluster core masses derived from x-ray observations differ from those determined from strong gravitational lensing.
P. J. Green (SAO), D. Perley, W.A. Barkhouse, R.A. Cameron, D.W. Kim, P. Maksym, J. Silverman, A.A. Vikhlinin, B. Wilkes, & ChaMP collaboration
We present preliminary results from a multi-wavelength search for galaxy groups and clusters imaged in x rays by Chandra and in the optical by the Chandra Multi-wavelength Project (ChaMP). The ChaMP's x-ray pipeline includes automatic extended-source detection, which is augmented and verified by visual inspection. We extend the detection and verification of these galaxy associations to the optical via (1) Voronoi Tesselation and Percolation analysis of galaxy clustering and (2) early-type galaxy red sequences in color-magnitude diagrams constructed from 3-filter deep optical images obtained with the MOSAIC CCD at NOAO 4-meter telescopes. The deep sensitivity of our imaging, combined with the multi-wavelength approach, offers a diverse cluster sample probing both high redshifts and low luminosities. We highlight several new clusters detected by the ChaMP.
K. Gutierrez (Washington U, St. Louis), H. Krawczynski, & D. Harris
In a study of the northern sub-cluster of A115 in a 50-ks Chandra ACIS I exposure, we discovered several x-ray surface brightness jumps in the southwestern section of the sub-cluster. Examination of the surface-brightness profiles reveals that these discontinuities are real and can be modeled with a central sphere and three shells, each of constant emissivity. Although, the northern sub-cluster appears to be pushing through the intra-cluster medium (ICM), we find no evidence for a shock associated with any of these discontinuities. There is a pressure jump of 1.7�0-10 dyne cm2 between the first and second shell (innermost and next out); however, there is no corresponding temperature jump, the temperatures being the same 2.7 keV within both shells. This pressure jump gives an upper limit on the velocity of the inner shell of Mach 0.589 through the outer shell. The core of the cluster is significantly cooler (1.610.14 keV) than the outer shells (2.592.89 keV). The cooling time in the core (2.95108 yr) is significantly shorter than the Hubble time, confirming the presence of a strong cooling flow. Finally, we discuss the correlation of the x-ray surface brightness distribution with the morphology of the radio galaxy 3C 28.0 at the center of the northern sub-cluster.Back to Topics
F. Paerels (Columbia)
Evidence for the presence of hot, highly ionized gas in the intergalactic medium outside groups and clusters is starting to emerge from x-ray imaging, while high-resolution x-ray absorption spectroscopy has revealed the presence of hot gas in the Local Group and (possibly) beyond. All the available data seem to suggest that we are close to detecting a sizeable fraction of the missing baryons at small redshift, probably from the highest-overdensity non-virialized gas. I discuss the evidence and its implications, and outline the requirements for a definitive characterization of the hot intergalactic gas based on high-resolution x-ray emission and absorption spectroscopy.Back to Topics
G. Fabbiano (SAO)
Simulations of galaxy mergers and, more recently, the Hubble deep-field images have shown that interaction at all levels may constitute a significant step in the formation and evolution of galaxies. With Chandra, we can study nearby examples of galaxy interaction and merging, which provide a local laboratory for galaxy evolution. Merging results in luminous and spectacular x-ray emission, both from population of point-like sources and from hot plasmas in these systems. Here I review the Chandra results, with particular emphasis on recent data on the Antennae galaxies, the prototypical galaxy merger.
R.P. Kraft (SAO), G. Boue, W.R. Forman, C. Jones, S.S. Murray, E. Churazov, A.A. Vikhlinin, M. Markevitch, & S. Heinz
We present preliminary results from Chandra ACIS-I observations of 2 nearby pairs of merging early galaxies NGC 3607/3608 and NGC 7619/7626. We measured temperatures, elemental abundances, and surface-brightness profiles for each of the 4 galaxies. NGC 3607 is 3 times more luminous than NGC 3608 in the optical band, but the ratio of x-ray luminosity to optical luminosity is approximately the same in each. This is surprising because the gas temperatures are also the same ( 0.6 keV). This indicates that, while NGC 3607 contains 3 times the mass of gas and stars compared with NGC 3608, the depths of their gravitational potentials are similar. The second pair NGC 7619/7626 is perhaps more interesting and unusual. The average gas temperature of both galaxies is 1.2 keV, but NGC 7619 is the more x-ray luminous of the two. However, there is a sharp surface-brightness discontinuity in the emission from NGC 7619 in the direction of NGC 7626 and an extended x-ray tail in the opposite direction. This suggests that NGC 7619 is falling into NGC 7626, perhaps supersonically, and that the mass of NGC 7626 is considerably larger than that of NGC 7619. We estimate the gravitating mass of NGC 7626 to be roughly 5 times that of NGC 7619. This implies that either a considerable fraction of gas around NGC 7626 escaped at some time in the past or that the gas is not currently visible because it is contained in a hot, low-density halo. We speculate that much of the gas around NGC 7626 was heated or expelled during an earlier epoch of radio activity.
J.S. Kaastra (SRON), N. Arav, G. Kriss, M. Sako, E. Behar, A. Laor, K. Korista, F. Paerels, M. de Kool, D. Proga, & K. Steenbrugge
We present the first results from a long (350-ks) Chandra/FUSE/HST campaign on the Seyfert-1 galaxy Mrk 279. The data were obtained in 2003 May. The Chandra Low-Energy Transmission-Grating Spectrometer (LETGS) spectrum shows a warm absorber with particularly strong lines from C, N and O of several ionization stages. The warm absorber has a relative low ionization parameter. In addition, relatively strong Galactic foreground absorption lines are present. We discuss these results also in the context of the simultaneous UV data that we obtained for this galaxy.
A. Young (Maryland) & A. Wilson
We present sub-arcsecond imaging spectroscopy of the low-luminosity active galactic nucleus of NGC 4258. The spectrum of the nucleus is well described by a heavily absorbed, hard x-ray power law of variable luminosity, plus a constant, thermal soft x-ray component. We do not detect Fe-Ka emission with the upper limit to the equivalent width of a narrow, neutral Fe line ranging between 92 and 747 eV (90% confidence) for the different observations. During the second observation, two narrow absorption features are seen at >99.5% confidence at 6.4 keV and 6.9 keV, which we identify as resonant absorption lines of Fe XVIIIXIX Ka and Fe XXVI Ka, respectively. In addition, the 6.9-keV absorption line is probably variable on an approximately 6-ks time scale. We discuss the properties of the absorbing gas for both photo- and collisionally ionized models. Given that the maser disk is viewed at an inclination i = 82, the gas responsible for the 6.9-keV absorption line may be in an inner disk, a diskwind boundary layer, or thermal gas entrained at the base of the jet. The gas that gives rise to the photoelectric absorption may be the same as that which causes the 6.4-keV Fe-Ka absorption, provided that the gas has a bulk velocity dispersion of a few thousand km s-1. This is the first detection of Fe x-ray absorption lines in an extragalactic source with a nearly edge-on accretion disk. This phenomenon is likely to be related to similar x-ray absorption lines in Galactic x-ray binaries with nearly edge-on accretion disks.
R. Kilgard (SAO), J.J. Cowan, M.R. Garcia, P. Kaaret, M.I. Krauss, J.C. McDowell, A.H. Prestwich, F.A. Primini, C.J. Stockdale, G. Trinchieri, M.J. Ward, & A. Zezas
Emission from discrete point sources dominates the x-ray luminosity in spiral galaxies. We present results from a survey of 11 nearby, nearly face-on spiral galaxies using the Chandra X-ray Observatory. These galaxies span the Hubble sequence for spirals, allowing insights into the x-ray source population of many diverse systems. Here we give an overview of our survey, discuss x-ray luminosity functions and how they relate to star formation of the host galaxies, and discuss source colors and variability and what these can tell us about the composition of the x-ray source population. This work is partially supported by NASA contract NAS 8-39073 (CXC) and grant GO1-2092A.
A. Ptak (Johns Hopkins), C. Norman, G. Hasinger, A. Hornschemeier, J. Bergeron, A. Comastri, R. Giacconi, R. Gilli, K. Glazebrook,, T. Heckman, K. Kellermann, P. Rosati, G. Szokoly, P. Tozzi, J.-X. Wang, W. Zheng, & A. Zirm
We derive the cosmological star-formation rate in both the Chandra Deep Fields North and South from our x-ray luminosity function for galaxies (XLF) in these deep fields. The XLF is remarkably consistent with expectations from the infrared (IR) luminosity function for galaxies using the standard infrared-to-x-ray (IR/X) conversion factors. We also compare our results to those obtained from studying the Ha luminosity function. In the analysis, we make use of sophisticated statistical methods to classify the galaxies and the AGN, utilizing all data now available from radio, infrared, optical, and x-ray observations. We discuss the effects of AGN contamination in the sample. The x-ray-derived cosmic star-formation rate is consistent with estimates from other spectral bands up to redshifts of order unity. We discuss how x-ray studies may continue to be useful for probing the star-formation history of the universe by avoiding problems of obscuration. Consequences for future x-ray missions are discussed.
R. Di Stefano (SAO) & A.K.H. Kong
The selection of very soft x-ray sources from Chandra surveys of galaxies out to 20 Mpc reveals evidence for a class of sources with photon energies somewhat greater than those typical of luminous super-soft x-ray sources, but which can nevertheless be characterized by values of kT < 300 eV (Di Stefano \& Kong 2003 a, b, c; Kong \& Di Stefano 2003; Di Stefano et al. 2003 a, b). A promising model for these sources (and for some super-soft sources as well) is that they are accreting intermediate-mass black holes. We provide an overview of the discovery of more than 200 quasi-soft sources in over a dozen galaxies, describing the distributions of their luminosities and locations within galaxies, including in globular clusters. We focus on a theoretical model for the creation of these binaries and compare its predictions with the observations.
J. Irwin (Michigan), A, Athey, & J. Bregman
We searched for ultra-luminous x-ray sources (ULXs) in a sample of elliptical and S0 galaxies observed with Chandra. We find that the number of x-ray sources detected at a flux level corresponding to a 0.310 x-ray luminosity of 21039 ergs s-1 or greater is equal to the expected number of foreground/background objects. In addition, the spatial distribution of the sources is consistent with these high-flux sources being evenly distributed over the Chandra field of view rather than distributed like the optical light of the galaxies, strengthening the argument that they are unassociated with the galaxies. We also took the ULX candidate list of Colbert and Ptak and determined the spatial distribution of ULXs in early-type galaxies and late-type galaxies separately. While the spiral galaxy ULXs are clearly concentrated toward the center of the galaxies, the elliptical and S0 galaxy ULXs are distributed randomly over the ROSAT HRI. This again indicates that they are not associated with the galaxies themselves. Hence, we conclude that, with the exception of two very rare high-luminosity objects within globular clusters of the elliptical galaxy NGC1399, ULXs are generally not found within old stellar systems.
K.K Ghosh (USRA/NSSTC), D.A. Swartz, A.F. Tennant, & K. Wu
As part of our on-going observational and theoretical study of ultra-luminous x-ray sources (ULXs) in the Chandra archive of galaxies, we present results of multi-wavelength studies of several ULXs. Long-term x-ray flux and spectral monitoring of these objects have been carried out using the ROSAT and XMM-Newton observatories. We also report multi-wavelength analyses of the environments surrounding these ULXs and our search for optical counterparts based on archival Hubble, Digitized Sky Survey, UV, and radio data.
N.A. Levenson (Kentucky), K.A. Weaver, & T.M. Heckman
Observations of the Seyfert-2 and starburst galaxy NGC 7130 with the Chandra X-ray Observatory illustrate that both these phenomena contribute significantly to the galaxy's detectable x-ray emission. The active galactic nucleus (AGN) is strongly obscured, buried beneath a column density NH > 1024 cm-2; it is most evident through a prominent Fe-Ka emission line with equivalent width greater than 1 keV. The AGN accounts for most (70%) of the x rays at energy E > 4 keV, with the remainder due to spatially extended star formation. The soft x-ray emission is predominantly thermal, on both small and large scales. We attribute the thermal emission to stellar processes, which include a large-scale galactic wind. In total, the AGN is responsible for only half the observed x-ray luminosity of 31041 erg s-1 of this galaxy. Buried AGN like NGC 7130 are truly common; similar examples may contribute significantly to the cosmic x-ray background while remaining hidden at most other wavelengths.
A. Baldi (CfA), S. Pellegrini, G. Fabbiano, & D.W. Kim
We present an analysis of the XMM-Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero Galaxy (NGC 4594). We investigate possible explanations for its very sub-Eddington luminosity by complementing the x-ray results with high-angular-resolution observations in other bands. The x-ray investigation shows a hard (G = 1.89) and moderately absorbed (NH = 1.81021 cm-2) nuclear source of 1.51040 erg s-1 (210 keV), surrounded by hot gas at a temperature kT 0.6 keV. The bolometric nuclear luminosity is at least 200 times lower than expected if mass accreted on the supermassive black hole that HST shows to reside at the center of this galaxy, at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disk. The low luminosity, coupled to the observed absence of Fe-K emission in the nuclear spectrum, indicates that such a disk is not present. This nucleus differs from bright, unobscured AGNs also for the lack of high flux variability and of prominent broad Ha emission. However, it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate; it could be too radio bright, instead, for radiatively inefficient accretion that includes strong mass outflows or convection. This discrepancy could be solved by the possible presence of nuclear radio jets. An alternative explanation of the low luminosity, in place of radiative inefficiency, could be unsteady accretion. This research is supported in part by the CXC contract (NAS8-39073) and the XMM grant NAG5-9984.
A.F. Tennant (NASA/MSFC), D.A. Swartz, K.K. Ghosh, & K. Wu
NGC 6822 is a dwarf irregular member of the Local Group. It is a site of recent star formation, rich in HII regions and OB associations, as well as containing an older globular-cluster population. We present results of a Chandra observation of NGC 6822 that reached down to a luminosity of 11035 erg s-1. The brightest source is extended and most likely a SNR in NGC 6822. After removing Galactic and extragalactic background sources, we find that the galaxy contains about 10 sources. This is consistent with our expectations if we scale the low-mass x-ray binary (LMXB) population by the mass and the high-mass x-ray binary (HMXB) population by the star formation rate.
D.A. Swartz (USRA/NSSTC), K.K. Ghosh, A.F. Tennant, & K. Wu
We report the x-ray spatial, spectral, and temporal properties of a large number of ultra-luminous x-ray sources (ULXs) obtained from archived Chandra ACIS observations of galaxies. Correlations between ULXs and properties of their host galaxies show that ULXs occur more frequently in galaxies with recent star formation. The x-ray characteristics of ULXs are compared to those of less-luminous sources in the same fields. Apart from intrinsic brightness, it is found that ULXs are statistically indistinguishable from stellar-mass black holes and neutron stars based on the current data. In particular, there is no compelling evidence for intermediate-mass black holes in the sample. However, the preponderance of ULXs in star-forming and merging/interacting galaxies does suggest ULXs originate in a young but short-lived population such as the high-mass x-ray binaries (HMXBs).
N. Isobe (NASDA), K. Makishima, M. Tashiro, N. Iyomoto, H. Kaneda, M. Suzuki, M. Murakami, A. Keiichi, & M. Mori
Lobes of radio galaxies provide the ideal laboratories for investigating jet formation, because they contain the time-integrated information on the past jet activities. Electrons in the lobes, emitting synchrotron radiation (SR), can produce inverse Compton (IC) photons. In the lobes, the cosmic microwave background (CMB) usually dominates other candidates for seed photons. Because the CMB energy density is known, a comparison of SR and IC fluxes allows us to determine electron and magnetic-field energy densities, ue and um. Since we discovered these IC x rays from Fornax A with ASCA, we have detected them from a number of radio lobes utilizing ASCA, Chandra and XMM-Newton. We found two important implications. One is that ue tends to dominate um typically an order of magnitude. The other is that the total electron energy seems to be proportional to the x-ray luminosity of the nucleus, although the magnetic energy appears to be almost unchanged. Making the most of the unprecedented angular resolution of Chandra, we found that the distribution of IC x rays is relatively uniform over the lobes of 3C 452, despite a rim-brightening feature in the SR radio profile. This suggests that ue is uniformly distributed, while um becomes stronger toward the periphery. We have found the same tendency with ASCA in Fornax A. These properties strongly imply that the electrons play a more important role than the magnetic fields in the evolution of the lobes, and perhaps in jet formation as well; however, a larger sample is needed.
J. Zeng (Maryland), I.M. George, & S.B. Kraemer
We present preliminary results from a search for extended x-ray emission surrounding several nearby Seyfert-11.5 galaxies using the Chandra X-ray Observatory. Such emission is detected in several cases, after careful consideration of the nuclear emission and point-spread function. We briefly discuss the energy dependence and likely origin(s) of the emission. We also compare the x-ray morphology to other orientation indicators, such as those obtained from the radio and [O III].
A. Kong (CfA) & R. Di Stefano
The selection of very soft x-ray sources from Chandra surveys of galaxies out to 20 Mpc reveals evidence for a class of sources with photon energies somewhat greater than those typical of luminous super-soft x-ray sources (SSSs), but which can nevertheless be characterized by values of kT < 300 eV, or SSS-like spectra with some harder x-ray emission (Di Stefano & Kong 2003a,b,c; Kong & Di Stefano 2003; Di Stefano et al. 2003 a,b). We refer to these as quasi-soft sources (QSSs). One physical model is that luminous QSSs and SSSs have a black-hole accretor, possibly an intermediate-mass black hole. We present the physical properties of some individual QSSs and SSSs observed with Chandra and/or XMM-Newton, including x-ray spectra, light curves, and possible optical counterparts. These can then be compared with predictions.
R. Iizuka (ISAS), Y. Maeda, Y. Terashima, & H. Kunieda
NGC 4388 is an edge-on spiral galaxy harboring a Seyfert-2 nucleus. This galaxy is located 1 away from the center of the Virgo Cluster, and the interaction between this galaxy and the Virgo intra-cluster medium (ICM) has been suggested. We analyzed Chandra data of NGC 4388 to search for signature of the interaction. We found some x-ray gas clouds, 1030 kpc away from the nucleus in a Chandra image in the 0.32.0 keV band. All the clouds are located northeast of the galaxy; some have been already reported by Iwasawa et al. (2003). One of the clouds is located 30 kpc from the nucleus. In addition, a hint of an arc-like structure connecting this region and the nucleus is seen. Ha emission with a very large extent of about 35 kpc has been reported around NGC 4388 (Yoshida et al. 2002, 2003). The distribution of the extended x-ray emission roughly follows that of Ha, but is slightly shifted from that of Ha. We combine our x-ray result, optical morphologies, and optical spectra (velocities and ionization states), and discuss the origin of the extended x-ray emission.
K. Wu (London), A.F. Tennant, D.A. Swartz, K.K. Ghosh, & R. Hunstead
Discrete x-ray sources in galaxies are found to show various types of luminosity functions. In general, simple power-law luminosity functions are found in starburst galaxies or disks of spiral galaxies, in which star formation is occurring; broken-power-law luminosity functions, in elliptical galaxies or bulges of spiral galaxies. We argue that aging of the x-ray binary population can cause the breaks in the luminosity function. Aging also leads to the domination of lower mass x-ray binaries, among which many are transients. We construct an analytic model to explain the evolution of x-ray source population in normal galaxies. The model also takes into account stochastic effects, such as transient properties of the sources and the duty cycles of the x-ray phase and binary capture in globular clusters. The results obtained from the model constitute a basis for interpreting the Chandra observations of statistical properties of discrete sources in nearby galaxies. The model provides the relevant essential physics for a more complicated, multi-dimensional parameter-space population synthesis.
D.W. Kim (SAO) & G. Fabbiano
Using Chandra archival data, we determined the x-ray luminosity function (XLF) for a sample of 14 early-type galaxies with a total of about 1000 point sources. After carefully correcting for sample incompleteness at faint Lx, we find that the apparent XLF flattening at low luminosities disappears. A single power-law model with a slope 2 (in differential form) is only marginally rejected. Adding a Gaussian bump or applying a broken power-law marginally improves the goodness of fit. The location of the bump or the break luminosity (if it exists) is 41038 erg s-1, much higher than the Eddington luminosity of a 1.4-Msun neutron star. For a single power law plus bump, the best-fit power-law slope is 2.10.1 and the contribution of the bump is just a few percent of LX(LMXB). For a broken-power-law distribution, the best-fit power-law slopes are 1.80.1 and 2.70.5 below and above the break luminosity, respectively. We note that, even if the XLF breaks, it is not as flat below the break luminosity as previously believed. This implies that a considerable number of point sources remain undetectable even in the Chandra observations. We estimate the total Lx(LMXB) to be (0.90.5)�030 erg s-1 / Lsun(B), if the XLF slope continues down to with Lx(min) = 1037 erg s-1. There is a weak correlation between Lx(LMXB)/L(B) and the specific frequency of globular clusters, indicating a possible GCLMXB connection. However, the scatter remains statistically significant. We emphasize the proper consideration of the hidden population of LMXBs is critical in measuring metal abundances of the hot ISM. This work has been partly supported by the CXC contract, NASA NAS8-39073.
T. Roberts (Leicester), R. Warwick, M. Ward, & M. Goad
We report the results of a program of dual-epoch Chandra observations of 5 ultra-luminous x-ray sources (ULXs) in nearby galaxies. Our data reveal a variety of temporal and spectral characteristics, with distinct changes in the sources between separate epochs. The behavior shown by the sources and the range of physical parameters are consistent with mildly beamed x-ray binaries, though we cannot rule out intermediate-mass black holes in any specific case. Highlights include the detection of a very luminous thermal-plasma component in one ULX that was not present in its x-ray spectrum 5 months earlier. We also present results of optical integral-field spectroscopy for these ULX, showing that the majority of them appear to reside within, or close to, a region of active star formation. We find this to be a common feature for ULXs in nearby normal spiral galaxies.
A. Dobrzycki (ESO/Garching), P.J. Groot, L.M. Macri, & K.Z. Stanek
We present optical identifications of approximately 100 Chandra x-ray sources in the direction of the Magellanic Clouds. The sources were found during an ongoing project searching for quasars behind the Clouds (Dobrzycki et al. 2002, 2003ab), via matching serendipitous Chandra sources with objects from the OGLE-II database (Udalski et al. 1998; Zebrun et al. 2001). Application of this method recently led to discoveries of several quasars behind the inner, dense parts of the Clouds, where very few quasars had been known until very recently. The efficiency of the quasar search is slightly higher than 10%, the remaining sources being primarily x-ray binaries and early-type stars. We present identifications and spectral and timing properties of the objects in both x-ray and optical bands. We discuss the application of the objects for follow-up studies and also prospects for quasars searches behind other dense stellar fields and possible application of objects for other projects, such as distance determination to the galaxies. This research was supported in part by NASA contract NAS8-39073.Back to Topics
K.A. Weaver (NASA/GSFC)
Since their respective launches in 1999, the Chandra and XMM-Newton satellites have spent a significant amount of observing time on classic studies of active galactic nuclei (AGN). As an example, Chandra has performed approximately 1000 observations of AGN, constituting about 23% of the total observing time for the mission. These observations have revealed a deeper understanding of previously studied phenomena such as ionized absorbers and iron-K profiles from accretion disks and have made many new and remarkable discoveries such as the imaging of AGN jets and insights into the starburst-AGN connection. I review several important and recent contributions to the field of AGN studies that have been provided by Chandra and XMM-Newton.
R.D. Blandford (KIPAC/Stanford)
I review the phenomenology of jets in black-hole systems and describe models for jets and their formation, highlighting key theoretical questions. Finally, I discuss advances in our understanding of jets through x-ray observations.
T.J. Turner (NASA/GSFC, UMBC), S.B. Kraemer, & J.N. Reeves
Arguably the most surprising new result from x-ray grating spectroscopy of AGN has been the discovery of narrow, redshifted components of Fe-Ka emission, first found in simultaneous Chandra HETGS and XMM-Newton EPIC spectra of NGC 3516. Further examples of the phenomenon have emerged in the literature since that discovery. We discuss our new results for Mkn 766, which shows a rapid shift in energy for one such emission line. We discuss new insight drawn from these data.
D.A. Schwartz (SAO/CfA), H.L. Marshall, J. Gelbord, D.M. Worrall, M. Birkinshaw, J. Lovell, D. Jauncey, E. Perlman, & D. Murphy
The x-ray study of radio jets is possible only with the exquisite angular resolution of the Chandra X-ray Observatory. In the context of quasar jets, the high-resolution x-ray data combined with the radio data allow measurement of the relativistic beaming factor of the jet. Upon analyzing 10 distinct regions within 4 jets detected in a snapshot survey; we find Doppler factors of 3 to 10. The kinetic flux carried by the jets is comparable to the bolometric accretion luminosity and considerably exceed it if the jets are not a pure electron/positron plasma. We consider the consequences of the jets kinetic flux not being limited to the Eddington luminosity of the central black hole. This research has been funded in part by NASA contract NAS8-39073 (SAO), and SAO SV1-61010 (MIT), and NASA grant GO2-3151C (SAO).
D. Chelouche (Tel-Aviv), H. Netzer, S. Kaspi, E. Behar, W.N. Brandt, I.M. George, D.M. Crenshaw, J.R. Gabel, F.W. Hamann, S.B. Kraemer, G.A. Kriss, K. Nandra, B.M. Peterson, J.C. Shields, & T.J. Turner
We present a detailed analysis of the 90-ks spectrum of NGC3783 obtained by Chandra HETGS in 20012002. We split the data in various ways to look for time-dependent and luminosity-dependent spectral variations. This analysis, combined with state-of-the-art photo-ionization calculations, leads to the following conclusions: (1) Luminosity variations on short (2-d) time scales are not associated with significant spectral variations. (2) On longer time scales ( 50 d), we discover two very different spectral shapes corresponding to the appearance and disappearance of a soft x-ray continuum component. These are not related in a simple way to the variations of the hard x-ray flux of the object. (3) The data are consistent with no changes in the absorbers properties. (4) Photo-ionization analysis reveals the presence of at least 3 absorbing components each with very different density and ionization but about the same gas pressure. The lower limits on their distances from the center are 0.2-3.2 pc.
D.A. Evans (SAO), R.P. Kraft, D.M. Worrall, M.J. Hardcastle, S.S. Murray, & C. Jones
Centaurus A is the brightest AGN in the hard-x-ray sky, with its emission dominated by the compact nucleus. The x-ray spectrum is complex and there are two schools of thought about its interpretation: One explains it in terms of synchrotron and Compton-scattered radiation from a compact jet; the other emphasizes the detection of Fe-K emission and models the emission in terms of accretion processes. We present results from Chandra HETGS and XMM-Newton EPIC observations that clarify the situation. The x-ray spectrum comprises both a heavily absorbed power-law component that we relate to the inner accretion phenomena, and a second, less absorbed, power-law component that we relate to emission from an inner radio jet. A strong Fe-Ka line is detected and resolved with the HETGS: It is consistent with fluorescence from neutral material. Unresolved Si-K line emission at 1.74 keV from neutral Si is also detected. We report on the spectral and variability properties of the source that support our interpretation of the emission processes.
E.C. Moran (Wesleyan)
It is now clear that the majority of the hard-x-ray background is produced by obscured AGNs at moderate redshift. In the local universe, these objects have Seyfert-2 optical spectra dominated by strong, narrow emission lines. The deep-survey results, identifying a large fraction of faint hard-x-ray sources with optically normal rather than Seyfert-2 galaxies, have thus provided one of the major surprises of the Chandra era. However, the quality and variety of data typically available for nearby and distant AGNs differ vastly, which makes it difficult to compare them properly. Bearing this in mind, I am investigating the extent to which local obscured AGNs are able to describe the properties of the distant hard-x-ray galaxy population. Simulating how nearby Seyfert-2 galaxies would appear at moderate redshifts in the 2-Ms Chandra Deep Field North (CDF-N), I find that the local objects successfully reproduce the observed 28-keV / I-band flux ratios of CDF-N sources with Seyfert-2-like x-ray characteristics. Differences in their optical spectra might be a simple consequence of the limitations of ground-based observing. Only integrated spectra are available for distant x-ray galaxies, which include emission from the nucleus as well as a great deal of extra-nuclear light from the host galaxy. I show that the integrated spectra of many nearby Seyfert-2 galaxies appear normal. Therefore, I conclude that it is too soon to abandon familiar Seyfert-2 galaxies as the objects in the deep fields with flat hard-x-ray spectra and thus the dominant contributors to the hard-x-ray background. I describe an approved HST program that will afford a direct test by providing high-angular-resolution nuclear spectra of a sample of z 0.2 normal hard-x-ray galaxies.
M.A. Sobolewska (SAO), A. Siemiginowska, & P.T. Zycki
We present spectral fits to data of high-redshift radio-quiet quasars (RQQs) observed with Chandra. We compute spectra in two geometries (1) a central, spherical hot flow surrounded by an accretion disk; and (2) hot clouds above an accretion disk (patchy corona). We find that in order to obtain spectra with the observed x-ray photon index, x-ray loudness, and optical/UV luminosity, the Comptonizing-plasma temperature kT 150 keV, implying low optical depths. In the case of the spherical flow, we find that the disk truncation radius 40 RS. For the patchy corona, the angular size of the Comptonizing clouds as seen from the accretion disk should be small (cos(q) 0.8-0.9). The clouds dissipate 10-60% of gravitational energy, intercept only about 10% of the viscous-disk radiation, and can move at mildly relativistic velocities. This research is funded in part by NASA contract NAS8-39073, Chandra GO1-2117B and GO2-3148A, and HST-GO-09280.01-A.
X. Dai (Penn State), G. Chartas, M. Eracleous, & G.P. Garmire
We present results from a mini-survey of moderately-high-redshift (1.7 < z < 4), gravitationally lensed radio-quiet quasars observed with Chandra and XMM-Newton. The lensing magnification effect allows searching for changes in quasar properties such as the evolution of the accretion process and x-ray flux variability over 3 orders of magnitude in intrinsic x-ray luminosity. We present spectral and temporal analyses of the x-ray properties of quasars at redshifts near the peak of their comoving number density, thought to have occurred at z 2. We find a possible correlation between the x-ray spectral slope and x-ray luminosity of the gravitationally lensed quasar sample. The x-ray spectral slope steepens as the x-ray luminosity increases. The optical-x-ray spectral index is found to depend upon rest-frame ultraviolet luminosity of the lensed sample. The upper limits of the x-ray variability of our moderately-high-redshift sample of lensed quasars is consistent with the known correlation between variability and luminosity observed in Seyfert-1 galaxies when extrapolated to the larger luminosities of our sample. We acknowledge support by NASA grant NAS 8-01128.
D. Donato (George Mason), R. Sambruna, & M. Gliozzi
In the framework of unified models, a supermassive black hole, an accretion disk, broad-line and narrow-line regions, a torus, and jets characterize radio-loud AGN. Recent optical and UV studies of Fanaroff-Riley Type-I (FR-I) radio galaxies belonging to the 3CR and B2 catalogs suggest the lack of an obscuring torus and thermal-disk emission. X rays are a powerful tool to probe the inner region of AGN and, therefore, to test these results. We investigate the origin of the x-ray emission from the nuclear region of FR-Is in the 3CR and B2 catalogs, using currently available Chandra and XMM-Newton observations. The combination of XMM-Newton and Chandra, with their complementary capabilities, is ideal for studying the complex x-ray emission of these sources.
G. Chartas (Penn State), M. Eracleous, E. Agol, S.C. Gallagher, & W.N. Brandt
We present new results from a re-analysis of a Chandra observation of the gravitationally lensed low-ionization broad-absorption-line (LoBAL) quasar H 1413+117. A previous analysis of the same Chandra observation led to the detection of a strong redshifted Fe-K line from the combined spectrum of all images. We show that the redshifted Fe-K line is only significant in the brighter image A. The x-ray flux fraction of image A is larger by a factor of 1.640.18 than the optical flux fraction, indicating that A is significantly enhanced in x rays. We also find that the enhancement is energy-dependent within the x-ray band. A micro-lensing event could explain both the energy-dependent magnification and the significant detection of Fe-K line emission in the spectrum of image A only. Assuming this interpretation, we provide constraints on the spatial extent of the scattered continuum and reprocessed Fe-Ka emission regions in a LoBAL quasar. Recent ray-trace modeling performed (Popovic et al. 2003) aimed at studying the influence of micro-lensing on the shape of the Fe-Ka line in quasars can reproduce the observed enhancement of the iron line in image A. H 1413+117 is a unique laboratory to test BAL flow models. The iron line serves as a probe of the kinematic structure of the flow.
M. Blazejowski (CfA), A. Siemiginowska, R. Moderski, & J. Bechtold
Chandra observations reveal very hard spectra in the luminous (Lx 1045 erg s-1), high-redshift quasars. Small optical-x-ray flux ratios, characterized by ao,x 1.0, suggest that the x rays result from beamed, nonthermal radiation. We model the x-ray emission in terms of the blazar phenomenon e.g., seed IR photons Compton scattered off relativistic particles in the parsec-scale jet. We discuss the range of model parameters appropriate for this sample. This research is funded in part by NASA contract NAS8-39073, Chandra GO2-3148A, and HST-GO-09280.01-A.
R. Dudik (George Mason), S. Satyapal, & R.M. Sambruna
Virtually all, extensive multi-wavelength studies of LINER galaxies have used optically-selected samples. Neglected in this pursuit has been a systematic multi-wavelength investigation of IR-bright galaxies, which constitute about 80% of all nearby LINERs. Here we present preliminary results from an x-ray snapshot survey using Chandra images of 16 nearby IR-bright LINER galaxies. The high angular resolution of the Chandra ACIS-S detector provides an ideal porthole to heavily obscured nuclei and, as a result, affords a valuable AGN diagnostic for these regions. We find hard nuclear point sources morphologically consistent with AGN in several sources. In addition, x-ray morphologies show a clear trend with IR brightness. This work confirms our previous study, which shows that most LINERs with a single, dominant, hard compact x-ray core are IR-faint, whereas most LINERs with scattered x-ray sources are IR-bright. This project complements and extends other projects aimed at studies of optically-selected or more luminous LINERs, allowing us to explore possible trends with luminosity or IR-brightness for a comprehensive understanding of this important class of objects. This work was supported by NASA Grant NAG03-4134X.
J.K. Gambill (George Mason), R.M. Sambruna, L. Maraschi, F. Tavecchio, & C.C. Cheung
We present Chandra and HST observations (PI Sambruna, A04) for PKS 1136-135 (81 ks) and 1150+497 (70 ks). These objects are originally from our multi-wavelength survey of 17 radio jets in AGNs with Chandra and HST (PI Sambruna, A02, 10 ks per target). Our snapshot exposure of PKS 1135-136 reveals different emission mechanisms along the jet, in addition to a disparity between the radio, optical, and x-ray morphologies. However, 1150+497 shows strong x-ray and optical emission that closely traces the radio-jet structure, with its oscillatory changes in position angle along the length of the jet. The complex multi-wavelength morphologies of these 2 jets raises new important questions about the mechanisms and physical conditions in AGN jets namely, whether jets are homogenous or inhomogeneous structures. We present here the detailed spectral and topographical analysis of the x-ray and optical jets observed in these 2 objects, including high-resolution images of these spectacular jets. Analysis of the Chandra and HST data provides reliable spectra of the individual x-ray knots of the 2 jets, discriminating between emission models (IC/CMB and SSC) and probes changing physical conditions along the jet.
K.-I. Nishikawa (NRC/NSSTC), P. Hardee, G. Richardson, R. Preece, H. Sol, & G.J. Fishman
Shock acceleration is ubiquitous in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman, two-streaming, and Weibel instabilities) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we investigated particle acceleration associated with a relativistic jet front propagating through ambient plasma, with and without initial magnetic fields. We find only small differences in the results between no-ambient and weak-ambient magnetic fields. Simulations show that the Weibel instability (in the collisionless shock front) accelerates particles perpendicular and parallel to the jets propagation direction. While some Fermi acceleration may occur at the jet front, the majority of electron acceleration occurs behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that this instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields, which contribute to the electron's transverse deflection behind the jet head. The jitter radiation (Medvedev 2000) from deflected electrons has different properties than synchrotron radiation from a uniform magnetic field. This jitter radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.
M. Karovska (SAO)
We highlight results from our study of multi-scale spatial structures in Chandra observations of several AGN using imaging techniques. The restored images reveal multi-scale (including sub-arcsecond) structures in the vicinity of the nuclei. The Chandra High-Resolution Mirror Assembly (HRMA) point spread function (PSF), combined with the small pixel sizes of the detectors and with nearly negligible aspect blurring, produces sharper images then any other x-ray telescope to date. However, the size and the shape of the PSF change significantly with off-axis angle and with the spectral distribution of the source. Hence, it is difficult to access the high-resolution spatial structures in many sources. Therefore, accurate knowledge of the PSF is crucial to multi-scale analysis of observed sources. We describe techniques that allow us to access the multi-scale structures in several AGN especially the small-scale spatial structures in these sources. These techniques include modeling Chandra HRMA and detectors PSFs, adaptive smoothing, and de-convolution techniques.
A. De Rosa (IASF/Rome) & L. Piro
We present a study of a sample of Seyfert-1 galaxies observed by BeppoSAX with a long-look strategy about 200-ks exposures for each source. The broad spectral coverage (0.1300 keV) and good energy resolution (8% at 6 keV) allow us to disentangle unambiguously all contributing spectral components warm absorber(s), soft excess, Compton reflection, and primary continuum. Doing so, we measure simultaneously their variations and search for correlated variability linking different components to a common origin, obtaining several notable results. Regarding the primary continuum, we find it to well described by a power law up to about 200 keV, where a high-energy cut-off occurs. In addition, the continuum shows systematic changes of spectral slope, steepening with increasing luminosity. We discuss these results and show that they are consistent with a thermal-Comptonization scenario. We find that reflection features Compton hump and Fe-Ka are commonplace in our sample of Seyfert galaxies. However, their properties are not immediately consistent with a cold accretion disk. We investigate the possibility that the disk is photo-ionized by the hard x-ray photons. Finally, we find evidence for an additional Fe-Ka component a narrow-core profile, with a constant intensity even during large continuum variations. We discuss these results with respect to recent findings from XMM-Newton observations.
A. Steffen (Wisconsin), A. Barger, L. Cowie, R. Mushotzky, & Y. Yang
We investigate how the fraction of broad-line sources in the AGN population changes with x-ray luminosity and redshift. We use the Chandra Deep-FieldNorth 2-Ms survey and our Lockman-HoleNorthwest wide-area Chandra survey to construct the hard-x-ray luminosity function (HXLF) between z = 0.10.5 and z = 0.51. Comparing with the local HXLF measured from HEAO-1 data and the high-redshift HXLF measured from soft ROSAT and Chandra data, we find that the number density of sources with quasar-like luminosity steadily declines with decreasing redshift, while the number density of sources with Seyfert-like luminosities peaks at z = 0.51. Strikingly, however, the number density of broad-line AGNs remains roughly constant with redshift, while their average luminosity decline at lower redshifts, showing another example of cosmic downsizing.Back to Topics
G. Hasinger (MPE) & CDF-S Team
Deep x-ray surveys have shown that the cosmic x-ray background (XRB) is largely due to accretion onto supermassive black holes, integrated over cosmic time. These surveys have resolved more than 80% of the 0.110-keV x-ray background into discrete sources. Optical spectroscopic and photometric identifications show that the sources producing the bulk of the x-ray background are a mixture of unobscured (type-1) and obscured (type-2) AGNs, as predicted by the XRB population synthesis models. A class of highly luminous type-2 AGN, so called QSO-2s, has been detected in the deepest Chandra and XMM-Newton surveys. A new x-ray luminosity function of type-1 AGN is presented, derived from a combination of the deepest Chandra surveys in the Hubble Deep Survey North (HDF-N) and the Chandra Deep Field South (CDF-S) with shallower and wider ROSAT surveys. It is shown that low-luminosity AGN have a slower cosmological evolution, saturating at lower redshift, compared to the canonical redshift peak of high-luminosity quasars at z = 23, confirming the need for luminosity-dependent density evolution. The exciting future prospects of new Chandra and XMM-Newton deep-wide surveys are discussed.
A. Hornschemeier (Johns Hopkins) CDF-N Team, & GOODS Team
We review progress in ultra-deep x-ray imaging surveys of the (2-Ms) Chandra Deep Field North (CDF-N) and the (1-Ms) Chandra Deep Field South (CDF-S). These very-long observations allow high-energy-astrophysics studies across orders of magnitude in dynamical scale, from accretion onto supermassive black holes to individual star-forming regions within galaxies (and even coronal x-ray emission from Galactic stars). Here we highlight the latest results from these deep surveys, including results on both fields from the recently completed HST Advanced Camera for Surveys observing campaign the Great Observatories Origins Deep Survey (GOODS). Finally, we discuss exciting future prospects from the approved Extended CDF-S program, to be carried out during Chandras fifth year.
A. Koekemoer (STScI), D. Alexander, F. Bauer, J. Bergeron, W. N. Brandt, E. Chatzichristou, S. Cristiani, S. M. Fall, N. Grogin, M. Livio, V. Mainieri, L. Moustakas, P. Padovani, P. Rosati, E. Schreier, & C.M. Urry
Extreme x-ray/optical-ratio sources (EXOs) are a possible new class of source. They are robustly detected in the Chandra Deep Field x-ray data and in deep near-IR VLT imaging, but remain completely undetected in our deep multi-band Great Observatories Origins Deep Survey (GOODS) HST/ACS data, including the long-wavelength z-band data. These sources have values of (Fx/Fo) that are 10100 times higher than those generally found for other AGNs. We thus infer two plausible scenarios. (1) If these sources lie at moderately high redshifts (z = 35), then their hosts need to be exceedingly under-luminous and/or excessively reddened compared with other AGN hosts. (2) If they lie above z > 7, such that their Lya emission is redshifted out of the z-band filter, then their K-band and x-ray properties can be accounted for in terms of relatively normal hosts and moderate-luminosity AGNs. In this case, these objects can serve as an invaluable probe of black-hole growth and accretion activity in the early universe.
Y. Ueda (ISAS), M. Akiyama, K. Ohta, & T. Miyaji
We investigate cosmological evolution of the hard-x-ray luminosity function (HXLF) of active galactic nuclei (AGNs) in the 210-keV luminosity range 1041.546.5 erg s-1 as a function of redshift for z 3. Combining surveys at photon energies above 2 keV conducted with Chandra, ASCA, and HEAO1, we construct a nearly (>96%) complete sample comprising 247 AGNs over the wide flux range 10-10 - 3.810-15 erg s-1 cm-2 (2-10 keV). For our purposes, we develop an extensive method of calculating the intrinsic (before-absorption) HXLF and the absorption (NH) function. This utilizes the maximum-likelihood method, fully correcting for observational biases with consideration of the x ray spectrum of each source. We find that (1) the fraction of x-ray absorbed AGNs decreases with the intrinsic luminosity and (2) the evolution of the HXLF of the entire AGN sample (including both type-I and type-II AGNs) is best described with a luminosity-dependent density evolution (LDDE) with a cut-off redshift increasing with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard-x-ray background, thus solving its origin quantitatively. A combination of the HXLF and the NH function enables us to construct a purely observation-based population-synthesis model. We present basic consequences of this model and discuss the contribution of Compton-thick AGNs to the rest of the hard x-ray background.
A.T. Kenter (SAO), W.R. Forman, C. Jones, S.S. Murray, A. Vihklinin, A. Dey, B.T. Jannuzi, J. Najita, B.R. McNamara, & J.C. Shields
We present preliminary results from Chandra observations of the Bootes 9-square-degree survey field. In 2003 March, the Chandra X-ray Observatory completed 126 separate ACIS-I observations, each about 5-ks duration. These observations, centered on right ascension aJ2000 14h32m and declination dJ2000 3406, were arranged in a mosaic with total field of view 9.3 square degrees. This unique set of observations allows calculation of x-ray source statistics over a wide, contiguous field of view, with unprecedented angular resolution and uniform coverage. In our full-band (0.5-7-keV) sample, we detect >2500 point and extended sources. Limiting the point-source list to sources with > 4 counts and assuming a simple AGN-type power-law spectrum, these observations reach a flux of about 410-15 ergs s-1 cm-2 (0.5-2 keV), 210-14 ergs s-1 cm-2 (2-10 keV), and 810-15 ergs s-1 cm-2 (5-10 keV). We present number of sources versus flux density differential NS(S) and integral N(>S) and hardness-ratio distributions; we then examine the distribution of x-ray sources for evidence of large-scale structure. In addition, we discuss our analysis techniques and related Monte-Carlo simulations. Funding for this research was provided in part by NASA contracts NAS8-38248 and NAS8-01130.
P. Gandhi (ESO/Santiago) & A.C. Fabian
The most intriguing results to emerge from deep Chandra and XMM-Newton surveys are (1) the large number of active galactic nuclei (AGN) with little sign of optical-near-infrared activity and (2) the relatively-recent peak in their redshift distribution at z 0.7. While the former observation probably implies that many AGN are completely enshrouded with dust, the latter reveals a resurgence in universal accretion activity for z < 2. We present the results of a model that successfully reproduces the peak in the space-density while also accounting for the spectrum of the x-ray background and the areal source counts as a function of flux. The main premise of the model is that obscured AGN activity proceeds in conjunction with the powerful star formation that is seen in the far infrared (FIR). While recent determinations of the hard-x-ray AGN luminosity function suggest no evolution in the fraction of absorbed AGN with redshift, we show that there is likely to be evolution if correction is made for redshift incompleteness to the faint flux levels of deep Chandra surveys.
Y. Yang (Maryland), R.F. Mushotzky, A.T. Steffen, A.J. Barger, & L.L. Cowie
One of the most interesting discoveries made from Chandra and XMM-Newton observations is that most of the x-ray sources that contribute to the x-ray background do not show activity in the optical bands. There has been evidence that this new population may be very different from active galaxies detected in optical surveys. We present the x-ray properties of the point source detected in the Chandra Large-Area Sensitive Survey of the Lockman Hole - Northwest region. This provides a large sample of the x-ray background sources at 2-8-keV flux threshold of 510-15 erg s-1 cm-2. The hardness ratio and variability of the sources are examined.
P. Ranalli (Bologna)
Thanks to the linear relations existing among radio, infrared, and x-ray luminosities of non-active spiral galaxies, we recently calibrated a new star-formation rate (SFR) indicator that is largely unaffected by absorption and has been tested in both local and high-redshift (z = 1) galaxies. Coupled with the deepest Chandra observations of the Chandra deep fields, this opens the possibility to put new constraints on the luminosity function of high-redshift star-forming galaxies and on the cosmic star-formation history (SFH). Here we present our last results for the x-ray number counts and emission of faint optically-selected galaxies in the Chandra Deep Field - South (CDF-S) area.Back to Topics
P.S. Broos (Penn State), L.K. Townsley, K. Getman, & F. Bauer
ACIS Extract (AE) is an IDL program that assists the observer in performing the many tasks involved in analyzing spectra of large numbers of point sources observed with ACIS. Notably, all tasks are performed in a context that may include multiple observations of the field. Features of AE and its several accessory tools include refining the accuracy of source positions; defining extraction regions based on the point spread function (PSF) of each source in each observation; generating single-observation and composite ARFs and RMFs; applying energy-dependent aperture corrections to the ARFs; computing light curves and KS tests for source variability; automated broadband photometry; automated spectral fitting and review of fitting results; and compilation of results into LaTeX tables. A variety of interactive plots are produced, showing various source properties across the catalog. Here we illustrate the capabilities of the package using the 4-pointing Rosette Molecular Cloud mosaic as an example field. The ACIS Extract code and a detailed users manual are available to the community at http://www.astro.psu.edu/xray/docs/TARA/ae_users_guide.html. Support for this effort was provided by the Chandra X-ray Observatory grant G01-2008X.
C.E. Grant (MIT) & M.W. Bautz
Over the lifetime of Chandra, the performance of ACIS continues to evolve. We present the four-year history of ACIS detector characteristics such as gain and charge transfer inefficiency to the present time and discuss expectations for the future. We also describe, in general terms, how this evolution affects science-data calibration.
M. Tsujimoto (Penn State), G. Chartas, K. Getman, P.S. Broos, L.K. Townsley, & E.D. Feigelson
We present a simple method to conduct spectral analysis of piled-up sources detected by an x-ray CCD (charge-coupled device). In this method, we discard all x-ray events at the core of the point spread function (PSF), which are both spatially and spectrally distorted due to pile-up, and use only events in the outskirt of the PSF. This method is particularly suitable for heavily piled-up sources (pile-up fraction > 10%), in which the existing pile-up model cannot be applied. We discuss how to create an appropriate set of response files and how to use simulations to evaluate whether the analysis well reproduces the original source spectrum. Finally, we apply this method to the piled-up sources detected in the Chandra Orion Ultra-deep Project study of Orion Nebula stars.
S.N. Virani (SAO), P.P. Plucinsky, D. Jerius, T. Gaetz, & D.T. Nguyen
We analyzed ACIS data of the external calibration source to characterize the grade-dependent spectral resolution of the ACIS S3 (back-illuminated) and I3 (front-illuminated) CCDs. We summarize our current understanding and discuss modeling of their behavior. We find that the computed pulse-height amplitude (PHA) for a given fluorescence-line energy is a function of the events grade. We investigate the positional dependence of this effect and how well the available charge-transfer-inefficiency (CTI) correctors mitigate it. Even modest improvements in spectral resolution could facilitate obtaining better spectral-fitting constraints for line-dominated plasmas, such as those found in supernova remnants. Therefore, we also discuss the impact on science-data analysis.
P.P. Plucinsky (SAO), A.A. Vikhlinin, H.L. Marshall, N.S. Schulz, R.J. Edgar, & M.T. Raley
The low-energy sensitivity of the ACIS instrument has been degrading due to accumulation of a layer of contamination on the ACIS filter and/or CCDs. This molecular film introduces a new, energy-dependent absorption into the ACIS response. The thickness of the layer has been increasing with time and the spatial distribution over the ACIS focal plane has been changing with time. We have used multiple observations of the supernova remnant 1E0102.2-7219 in the Small Magellanic Cloud, to verify the models for the spectral, temporal, and spatial dependence of the contamination layer. 1E0102.2-7219 has a soft, line-dominated spectrum, which is very sensitive to the additional absorption by the contamination layer. 1E0102.2-7219 has been observed many different times over the course of the mission and at several different locations on the ACIS I and S arrays, such that it may be used to verify the temporal and spatial dependence of the contamination model.Back to Topics