Sergey P. Trudolyubov

M31 Globular Cluster X-Ray Sources: XMM-Newton and Chandra Results

We present the results of an M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 arcmin2 of M31. We detected 43 X-ray sources coincident with GC candidates from various optical surveys. The inferred isotropic X-ray luminosities of GC sources lie between ~1035 and ~1039 ergs s-1 in the 0.3-10 keV energy band. The spectral properties of the 31 brightest sources in our sample were found to be similar to those of the low-mass X-ray binaries (LMXBs) located in the bulge and the GCs of the Milky Way. The spectral distribution of the M31 GC X-ray sources is consistent with that derived for the bulge of M31 and other nearby galaxies of different morphological type. Several sources demonstrate a correlation between the level of X-ray flux and the hardness of their energy spectrum reminiscent of the Galactic Z and atoll sources. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on timescales from days to years. The X-ray source RX J0043.2+4127, coincident with GC Bo 163, has been found to show recurrent transient outbursts with peak luminosities of ~1038 ergs s-1. Several sources in our sample show significant variability on a timescale of individual observations, ranging from aperiodic fluctuations to regular dipping. The X-ray luminosity function of GC sources is found to be significantly different from that of the point sources in the bulge and disk of M31. The luminosity distribution of M31 GC sources has ~10 times higher peak luminosity and a much higher fraction of bright sources than the Milky Way GC distribution. Six persistent sources in our sample (or ~14% of the total number) have luminosities exceeding 1038 ergs s-1 during all observations, and three other sources occasionally exceed that luminosity level. Our observations indicate that GC sources make the dominant contribution to the bright source counts in the areas of M31 covered by the survey: ~40% of the total number of sources with luminosities above 1037 ergs s-1 reside in GCs, with the fraction of GC sources rising to 67%-90% for luminosities above 1038 ergs s-1. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way but surprisingly close to that of early-type galaxies. We found that the brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that GCs hosting bright X-ray sources are optically brighter and more metal-rich than the rest of the M31 GCs, in agreement with previous studies. The brightest sources, with luminosities above ~1038 ergs s-1, show a tendency to reside in more metal-poor clusters. The remarkable similarities between the properties of the M31 GC X-ray sources and those of Galactic neutron star LMXBs lead us to expect most of the persistent M31 GC X-ray sources to be LMXB systems with neutron star primaries. However, the current X-ray spectral and timing data cannot rule out the possibility of finding active accreting black holes in our GC source sample.

On the Correlated X-Ray and Optical Evolution of SS Cygni

We have analyzed the variability and spectral evolution of the prototype dwarf nova system SS Cygni using RXTE data and AAVSO observations. A series of pointed RXTE/PCA observations allow us to trace the evolution of the X-ray spectrum of SS Cygni in unprecedented detail, while 6 years of optical AAVSO and RXTE/ASM light curves show long-term patterns. Employing a technique in which we stack the X-ray flux over multiple outbursts, phased according to the optical light curve, we investigate the outburst morphology. We find that the 3-12 keV X-ray flux is suppressed during optical outbursts, a behavior seen previously but only in a handful of cycles. The several outbursts of SS Cygni observed with the more sensitive RXTE/PCA also show a depression of the X-rays during optical outburst. We quantify the time lags between the optical and X-ray outbursts and the timescales of the X-ray recovery from outburst. The optical light curve of SS Cygni exhibits brief anomalous outbursts. During these events the hard X-rays and optical flux increase together. The long-term data suggest that the X-rays decline between outbursts. Our results are in general agreement with modified disk instability models, which invoke a two-component accretion flow consisting of a cool optically thick accretion disk truncated at an inner radius, and a quasi-spherical hot corona-like flow extending to the surface of the white dwarf. We discuss our results in the framework of one such model, involving the evaporation of the inner part of the optically thick accretion disk, proposed by Meyer & Meyer-Hofmeister.

The Discovery of a 2.78 Hour Periodic Modulation of the X-Ray Flux from Globular Cluster Source Bo 158 in M31

We report the discovery of periodic intensity dips in the X-ray source XMMU J004314.1+410724, in the globular cluster Bo 158 in M31. The X-ray flux was modulated by ~83% at a period of 2.78 hr (10,017 s) in an XMM-Newton observation taken 2002 January 6-7. The X-ray intensity dips show no energy dependence. We detected weaker dips with the same period in observations taken 2000 June 25 (XMM-Newton) and 1991 June 26 (ROSAT/PSPC). The amplitude of the modulation has been found to be anticorrelated with source X-ray flux: it becomes lower when the source intensity rises. The energy spectrum of Bo 158 was stable from observation to observation, with a characteristic cutoff at ~4-6 keV. The photoelectric absorption was consistent with the Galactic foreground value. No significant spectral changes were seen in the course of the dips. If the 2.78 hr cycle is the binary period of Bo 158, the system is highly compact, with a binary separation of ~1011 cm. The association of the source with a globular cluster, together with spectral parameters consistent with Galactic neutron star sources, suggests that X-rays are emitted by an accreting neutron star. The properties of Bo 158 are somewhat reminiscent of the Galactic X-ray sources exhibiting diplike modulations. We discuss two possible mechanisms explaining the energy-independent modulation observed in Bo 158: (1) the obscuration of the central source by highly ionized material that scatters X-rays out of the line of sight and (2) the partial covering of an extended source by an opaque absorber that occults varying fractions of the source.

On the X-Ray Source Luminosity Distributions in the Bulge and Disk of M31: First Results from the XMM-Newton Survey

We present luminosity distributions for the X-ray sources detected with XMM-Newton in the bulge and disk of the Andromeda galaxy (M31). The disk is clearly lacking the bright sources that dominate X-ray emission from the bulge. This is the first convincing evidence of a difference between bulge and disk X-ray populations in M31. Our results are in good qualitative agreement with the luminosity distributions for low- and high-mass X-ray binaries recently obtained by Grimm, Gilfanov, & Sunyaev for our Galaxy. The data presented here confirm that X-ray population of the disk of M31 is dominated by faint high-mass X-ray binary sources, while the bulge is populated with bright low-mass X-ray binaries.

OBSERVATIONS OF THE X-RAY AFTERGLOWS OF GRB 011211 AND GRB 001025 BY XMM-NEWTON

We present the XMM-Newton observations of X-ray afterglows of the gamma-ray bursts GRB 011211 and GRB 001025. For GRB 011211, XMM detected a fading X-ray object with an average flux in 0.2-10 keV declining from 2.7 × 10-13 ergs s-1 cm-2 during the first 5 ks of the 27 ks observation to 1.0 × 10-13 ergs s-1 cm-2 toward the end of the observation. The spectrum of the afterglow can be fitted to a power law with α = 2.16 ± 0.03 modified for the Galactic absorption. No significant evolution of spectral parameters has been detected during the observation. A similar X-ray spectrum with α = 2.01 ± 0.09 has been observed by the XMM from GRB 001025. The nondetection of any extra absorption in these spectra above the Galactic value is an interesting fact and may impose restrictions on the favorable GRB models involving the burst origin in star-forming regions. Finally, we discuss soft X-ray lines from GRB 011211 reported by Reeves et al. and conclude that there is no definitive evidence for the presence of these lines in the spectrum.

Observations of the Optical Counterpart to XTE J1118+480 during Outburst by the Robotic Optical Transient Search Experiment I Telescope

The X-ray nova XTE J1118+480 exhibited two outbursts in the early part of 2000. As detected by the Rossi X-Ray Timing Explorer (RXTE), the first outburst began in early January and the second began in early March. Routine imaging of the northern sky by the Robotic Optical Transient Search Experiment (ROTSE) shows the optical counterpart to XTE J1118+480 during both outbursts. These data include over 60 epochs from January to June 2000. A search of the ROTSE data archives reveal no previous optical outbursts of this source in selected data between 1998 April and 2000 January. While the X-ray-to-optical flux ratio of XTE J1118+480 was low during both outbursts, we suggest that they were full X-ray novae and not minioutbursts based on comparison with similar sources. The ROTSE measurements taken during the 2000 March outburst also indicate a rapid rise in the optical flux that preceded the X-ray emission measured by the RXTE by approximately 10 days. Using these results, we estimate a preoutburst accretion disk inner truncation radius of ~1.2 × 104 Schwarzschild radii.

Bright X-Ray Transients in the Andromeda Galaxy Observed with Chandra and XMM-Newton

We study three transient X-ray sources, that were bright in the central region of M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time in the history of X-ray astronomy, to build light curves of transient sources in M31 suitable for studying their variability on a time scale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral evolution. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+ 411759 is probably a classical nova-like system containing a magnetized, rapidly-spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 per year. The rate of the hard X-ray transients (~5 1/year) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.We study three transient X-ray sources that were bright in the central region of the M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time to derive X-ray light curves of transient sources in M31 suitable for studying their variability on a timescale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral characteristics. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+411759 is probably a classical nova-like system containing a magnetized, rapidly spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 yr-1. The rate of the bright hard X-ray transients (~5 yr-1) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.

XMM–Newton discovery of transient 285.4 s X-ray pulsar XMMU J013359.5+303634 in M33

I report on the discovery and analysis of the first transient X-ray pulsar detected in the Local Group galaxy M33. The 2010 July-August deep XMM-Newton observations of M33 fields revealed a new bright X-ray source XMMU J013359.5+303634 exhibiting pulsations with a period P~285.4 s and pulsed fraction ~47 per cent in the 0.3-10 keV energy range. The pulse phase averaged spectrum of XMMU J013359.5+303634 is typical of X-ray pulsars and can be fit with an absorbed simple power law model of photon index Gamma~1.2 in the 0.3-10 keV energy band. The search for an optical counterpart did not yield any stellar object brighter than 20 mag, suggesting that XMMU J013359.5+303634 is not a Galactic foreground object and almost certainly belongs to M33. Assuming the distance of 817 kpc, the maximum observed luminosity of the source in the 0.3-10 keV energy range is ~1.4e37 ergs/s, at least 20 times higher than quiescent luminosity. The brightest optical object inside the error circle of XMMU J013359.5+303634 has a visual magnitude of 20.9 and properties consistent with being an early B V star when placed at a distance of M33. Based on the X-ray pulsations and spectrum, transient behavior and possible early B class optical counterpart, XMMU J013359.5+303634 can be classified as another extragalactic Be/X-ray binary candidate.

Bright X-Ray Transients in the Andromeda Galaxy Observed with [ITAL]Chandra[/ITAL] and [ITAL]XMM-Newton[/ITAL]

We study three transient X-ray sources, that were bright in the central region of M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time in the history of X-ray astronomy, to build light curves of transient sources in M31 suitable for studying their variability on a time scale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral evolution. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+ 411759 is probably a classical nova-like system containing a magnetized, rapidly-spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 per year. The rate of the hard X-ray transients (~5 1/year) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.We study three transient X-ray sources that were bright in the central region of the M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time to derive X-ray light curves of transient sources in M31 suitable for studying their variability on a timescale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral characteristics. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+411759 is probably a classical nova-like system containing a magnetized, rapidly spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 yr-1. The rate of the bright hard X-ray transients (~5 yr-1) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.

Millenium year x-ray transients in andromeda galaxy

We study three transient X-ray sources, that were bright in the central region of M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time in the history of X-ray astronomy, to build light curves of transient sources in M31 suitable for studying their variability on a time scale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral evolution. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+ 411759 is probably a classical nova-like system containing a magnetized, rapidly-spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 per year. The rate of the hard X-ray transients (~5 1/year) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.We study three transient X-ray sources that were bright in the central region of the M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time to derive X-ray light curves of transient sources in M31 suitable for studying their variability on a timescale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral characteristics. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+411759 is probably a classical nova-like system containing a magnetized, rapidly spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 yr-1. The rate of the bright hard X-ray transients (~5 yr-1) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.