Albert K. H. Kong

The X-Ray Spectra of Black Hole X-Ray Novae in Quiescence as Measured by Chandra

We present Chandra observations of black hole X-ray novae V404 Cyg, A0620-00, GRO J1655-40, and XTE J1550-564 in quiescence. Their quiescent spectra can be well fitted by a power-law model with number slope α ~ 2. While a coronal (Raymond-Smith) model is also a statistically acceptable representation of the spectra, the best-fit temperatures of these models is ~5 times higher than that seen in active stellar coronae. These four spectra of quiescent X-ray novae are all consistent with that expected for accretion via an advection-dominated accretion flow and inconsistent with that expected from a stellar corona. This evidence for continued accretion in quiescence further strengthens the case for the existence of event horizons in black holes. Both A0620-00 and GRO J1655-40 were fainter than in previous observations, while V404 Cyg was more luminous and varied by a factor of 2 in a few kiloseconds. A reanalysis of the X-ray data for XTE J1550-564 shows that (like V404 Cyg and A0620-00) its luminosity exceeds the maximum prediction of the coronal model by a large factor. The 0.3-7 keV luminosities of the four sources studied are in the range from ~1030 to 1033 ergs s-1.

Luminous Supersoft X-Ray Sources in External Galaxies

We use a set of conservative criteria to identify luminous supersoft X-ray sources (SSSs) in external galaxies. We test this approach on blackbody models and then apply it to Chandra data from four galaxies: an elliptical galaxy (NGC 4967), two face-on spiral galaxies (M101 and M83), and an interacting galaxy (M51). We find SSSs in every galaxy and estimate that the total SSS population of sources with L > 1037 ergs s-1 in each galaxy is at least several hundred and could be significantly larger. In addition, we discover two intriguing features of galactic populations of SSSs. First, there are significant subpopulations of high-luminosity sources; the bolometric luminosities can exceed 1039 ergs s-1. Second, in the spiral galaxies M101, M83, and M51, SSSs appear to be associated with the spiral arms. This may indicate that some SSSs are young systems, possibly younger than 108 yr.

A SYNOPTIC X-RAY STUDY OF M31 WITH THE CHANDRA HIGH RESOLUTION CAMERA

We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of 2.5 years and contain a mean effective exposure of 17 ks. We have measured the mean fluxes and long-term lightcurves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well-fit by a power-law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. We note several sources in the southwestern disk with L_X > 10^{37} erg/s . We cross-correlate all of our sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. We analyze follow-up HST WFPC2 data of two X-ray transients, finding F336W (U-band equivalent) counterparts. In both cases, the counterparts are variable. In one case, the optical counterpart is transient with F336W = 22.3 +/- 0.1 mag. The X-ray and optical properties of this object are consistent with a ~10 solar mass black hole X-ray nova with an orbital period of ~20 days. In the other case, the optical counterpart varies between F336W = 20.82 +/- 0.06 mag and F336W = 21.11 +/- 0.02 mag. Ground-based and HST observations show this object is bright (V = 18.8 +/- 0.1) and slightly extended. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.

Bright X-Ray Sources in M31 Globular Clusters

We have conducted Chandra observations of � 2560 arcmin 2 (� 131 kpc 2 ) of M31 and find that the most luminous X-ray sources in most of our fields are in globular clusters. Of the 28 globular cluster X-ray sources in our fields, 15 are newly discovered. Approximately one-third of all the sources have LX([0.5–7] keV Þ > 10 37 ergs s � 1 , and approximately one-tenth of all the sources have LX([0.5–7] keV) close to or above 10 38 ergs s � 1 . The most luminous source, in the globular cluster Bo 375, is consistently observed to have LX greater than 2 � 10 38 ergs s � 1 . (1) We present data on the spectra and/or light curves of the five most luminous M31 globular cluster sources. (2) We explore possible explanations for the high X-ray luminosities of the brightest sources. These include that the X-ray sources may be composites, the radiation we receive may be beamed, metallicity effects could be at work, or the sources may be accreting black holes. We weigh each of these possibilities against the data. In addition, we introduce a neutron star model in which mass transfer proceeds on the thermal timescale of the donor star. Our model can produce luminosities of several times 10 38 ergs s � 1 and leads to a set of well-defined predictions. (3) We compute the X-ray luminosity function and the distribution of counts in wavebands that span the range of energies to which Chandra is sensitive. We find the peak X-ray luminosity is higher and that systems with LX > 10 37 ergs s � 1 constitute a larger fraction of all GC sources than in our Galaxy. (4) We study the possible reasons for this difference between M31 and Galactic globular cluster X-ray sources and identify three promising explanations. Subject headings: galaxies: individual (M31) — galaxies: star clusters — Galaxy: globular clusters: general — X-rays: galaxies — X-rays: stars

THE DISCOVERY OF QUASI-SOFT AND SUPERSOFT SOURCES IN EXTERNAL GALAXIES

We apply a uniform procedure to select very soft sources from point sources observed by Chandra in four galaxies. This sample includes one elliptical galaxy (NGC 4967), two face-on spiral galaxies (M101 and M83), and an interacting galaxy (M51). We report on some intriguing results, including the following: 1. We have found very soft X-ray sources (VSSs) in every galaxy. Some of these fit the criteria for canonical supersoft sources (SSSs), while others are somewhat harder. These latter have characteristic values of kT 300 eV; we refer to them as quasi-soft sources (QSSs). We found a combined total of 149 VSSs in the four galaxies we considered; 77 were SSSs and 72 were QSSs. 2. The data are consistent with the existence of a large VSS population, most of whose members we cannot observe because of the effects of distance and obscuration. The total VSS population of sources with L > 1037 ergs s-1 in each galaxy could be on the order of 1000. 3. Whereas in M31 only ~10% of all X-ray sources detected by Chandra are VSSs, more than 35% of all detectable X-ray sources in the face-on galaxy M101 fit the phenomenological definition of VSSs. This difference may be due to differences in NH between typical lines of sight to sources in each galaxy. 4. SSSs can be super-Eddington for Chandrasekhar-mass objects. 5. We find evidence for SSSs and QSSs with luminosities of 1036 ergs s-1 < L < 1037 ergs s-1. These sources have luminosities lower than those of the ~30 soft sources used to establish the class of SSSs. 6. In the spiral galaxies M101, M83, and M51, a large fraction of the SSSs and QSSs appear to be associated with the spiral arms. This may indicate that some SSSs are young systems, possibly younger than 108 yr. 7. In addition to finding hot white dwarfs and soft X-ray binaries, our method should also be efficient at selecting supernova remnants (SNRs). A small fraction of the VSSs in the spiral arms of M101 appear to be associated with SNRs

Correlated X-Ray and Optical Variability in V404 Cygni in Quiescence

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of 20 during a 60 ks observation. X-ray variations were well correlated with those in Hα, although the latter include an approximately constant component as well. Correlations can also be seen with the optical continuum, although these are less clear. We see no large lag between X-ray and optical line variations; this implies they are causally connected on short timescales. As in previous observations, Hα flares exhibit a double-peaked profile suggesting emission distributed across the accretion disk. The peak separation is consistent with material extending outward to at least the circularization radius. The prompt response in the entire Hα line confirms that the variability is powered by X-ray (and/or EUV) irradiation.

MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence*

We report on the analysis of new and previously published MMT optical spectra of the black hole binary XTE J1118+480 during the decline from the 2000 outburst to true quiescence. From cross-correlation with template stars, we measure the radial velocity of the secondary to derive a new spectroscopic ephemeris. The observations acquired during approach to quiescence confirm the earlier reported modulation in the centroid of the double-peaked Hα emission line. In addition, our data combined with the results presented by Zurita et al. (2002) provide support for a modulation with a periodicity in agreement with the expected precession period of the accretion disk of ~52 days. Doppler images during the decline phase of the Hα emission line show evidence for a hot spot and emission from the gas stream: the hot spot is observed to vary its position, which may be due to the precession of the disk. The data available during quiescence show that the centroid of the Hα emission line is offset by about -100 km s-1 from the systemic velocity, which suggests that the disk continues to precess. An Hα tomogram reveals emission from near the donor star after subtraction of the ringlike contribution from the accretion disk, which we attribute to chromospheric emission. No hot spot is present, suggesting that accretion from the secondary has stopped (or decreased significantly) during quiescence. Finally, a comparison is made with the black hole XRN GRO J0422+32: we show that the Hα profile of this system also exhibits a behavior consistent with a precessing disk.

Chandra studies of the X-ray point source luminosity functions of M31

Three different M31 disk fields, spanning a range of stellar populations, were observed by Chandra. We report the X-ray point source luminosity function (LF) of each region, and the LF of M31s globular clusters, and compare these with each other and with the LF of the galaxys bulge. To interpret the results we also consider tracers of the stellar population, such as OB associations and supernova remnants. We find differences in the LFs among the fields, but cannot definitively relate them to the stellar content of the fields. We find that stellar population information, average and maximum source luminosities, X-ray source densities, and slopes of the LF are useful in combination.

Evidence of an Intermediate-Mass Black Hole: Chandra and XMM-Newton Observations of the Ultraluminous Supersoft X-Ray Source in M101 during Its 2004 Outburst

We report the results of Chandra and XMM-Newton observations of a new outburst of an ultraluminous supersoft X-ray source in M101. CXOU J140332.3+542103 was observed in a low-luminosity state (LX ~ 1037 ergs s-1) between 2004 January and May. The low-state X-ray spectra were relatively hard; the combined low-state spectrum can be fitted with a combination of a power law with photon index of 1.4 and a blackbody of 63 eV. During 2004 July, the source underwent a strong outburst and the peak 0.3-7 keV luminosity reached 3 × 1040 ergs s-1, with a bolometric luminosity of about 1041 ergs s-1. The outburst spectra were very soft and can be generally fitted with a blackbody model with temperatures of 50-100 eV. In two of the observations, absorption edges at 0.33, 0.56, 0.66, and 0.88 keV were found. An XMM-Newton observation was also performed during the decay of the outburst, and a power-law tail was seen in addition to the supersoft spectrum. We consider different accretion models; one involving an intermediate-mass black hole can explain the observations.

X-Ray Point Sources in the Sombrero Galaxy: Very Soft Sources, the Globular Cluster/Low-Mass X-Ray Binary Connection, and an Overview

We report on the population of point sources discovered during an 18.5 ks Chandra ACIS-S observation of the Sombrero galaxy. We present the luminosity function and the spectra of the six brightest sources, consider correlations with globular clusters (GCs) and with planetary nebulae, and study the galaxys population of very soft sources. We detected 122 sources. Twenty-two sources are identified as very soft; of these, five appear to be classical luminous supersoft X-ray sources (SSSs), while 17 may belong to the slightly harder class referred to as quasi-soft (QSSs). There is an overdensity of very soft sources within 2 kpc of the nucleus, which is itself the brightest X-ray source. Very soft sources are also found in the disk and halo, with one QSS in a globular cluster (GC). This source is somewhat harder than most SSSs; the energy distribution of its photons is consistent with what is expected from an accreting intermediate-mass black hole. Several sources in the Sombreros halo are good candidates for SSS models in which the accretor is a nuclear-burning white dwarf. In total, 32 X-ray sources are associated with GCs. The majority of sources with luminosity greater than 1038 ergs s-1 are in GCs. These results for M104, an Sa galaxy, are similar to what has been found for elliptical galaxies and for the late-type spiral M31. We find that those optically bright GCs with X-ray sources house only the brightest X-ray sources. We find that, in common with other galaxies, there appears to be a positive connection between young (metal-rich) GCs and X-ray sources but that the brightest X-ray sources are equally likely to be in metal-poor GCs. The luminosity function of X-ray sources in GCs has a cut-off near the Eddington luminosity for a 1.4 M☉ object. We propose a model that can explain the trends seen in the data sets from the Sombrero and other galaxies. Thermal timescale mass transfer can occur in some of the younger clusters in which the turnoff mass is slightly greater than 0.8 M☉; multiplicity may play a role in some of the most massive clusters; accretion from giant stars may be the dominant mechanism in some older, less massive and less centrally concentrated clusters. Key elements of the model can be tested.