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.
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 phenomena
Back to TopicsJ.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 TopicsJ.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 TopicsU. 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 TopicsL.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 TopicsG.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 TopicsJ.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 TopicsS. 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 fe