RXTE observations of 4U 1630-47 during the peak of its 1998 outburst

We present an analysis of the RXTE observations of 4U 1630-47 during its outburst of 1998. The light curve and the spectral evolution of the outburst were distinctly different from the outbursts of the same source in 1996 and in 1999. Special emphasis of our analysis was on the observations taken during the initial rise of the flux and during the maximum of the outburst. The maximum of the outburst was divided into three plateaus, with almost constant flux within each plateau, and fast jumps between them. The spectral and timing parameters are stable for each individual plateau, but distinctly different between the plateaus. The variability detected on the first plateau is of special interest. During these observations the source exhibits quasi-regular modulations with period of ~10 - 20 s. Our analysis revealed significant differences in spectral and temporal behavior of the source at high and low fluxes during this period of time. The source behavior can be generally explained in the framework of the two-phase model of the accretion flow, involving a hot inner comptonization region and surrounding optically thick disk. The variability and spectral evolution of the source were similar to what was observed earlier for other X-ray Novae. We show that 4U 1630-47 has a variety of properties which are typical for Galactic black hole binaries, both transient and persistent. We argue that this system may be an intermediate case between different groups of black hole candidates.Comment: 14 pages, 12 figures, submitted to MNRA

On the Integrated Spectrum of the X-ray Binaries and the Origin of Soft X-ray Emission from the Bulge of M31

Using ROSAT PSPC data, we have performed several tests aimed at understanding the origin of the soft X-ray spectral component detected from the bulge of M31. We find that a significant soft component in the spectrum of the bulge is spatially correlated with the unresolved X-ray emission near the core of M31, which is probably a hot interstellar medium or perhaps a population of multiple faint sources. For the first time, we extracted the spectrum of this unresolved emission, by removing point sources dominating the integral spectrum of the bulge, and found it to be responsible for the most of soft excess. A soft spectral component is not at all needed to fit the point source spectrum that remains after subtracting the unresolved emission. The integral spectra of bright point sources, both inside and outside of the M31 bulge, can be fitted with a single power-law in the ROSAT band. Our analysis rules out the previous suggestion that all bulge emission in M31 may be generated by low mass X-ray binaries (Irwin & Bregman, 1999).Comment: 11 pages incl. 2 figures, 2 tables, accepted to ApJ

High-resolution Elemental Mapping of the Lunar Surface

New instruments and missions are being proposed to study the lunar surface as a result of the resurgence of interest in returning to the Moon. One instrument recently proposed is similar in concept to the x-ray fluorescence detectors flown on Apollo, but utilizes fluorescence from the L- and M-shells rather than the K-shell. This soft X-Ray Flourescence Imager (XRFI) is discussed

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.Comment: ApJ Letters, submitte

Fast X-ray Transients and Their Connection to Gamma-Ray Bursts

Fast X-ray transients (FXTs) with timescales from seconds to hours have been seen by numerous space instruments. We have assembled archival data from Ariel-5, HEAO-1 (A-1 and A-2), WATCH, ROSAT, and Einstein to produce a global fluence-frequency relationship for these events. Fitting the log N-log S distribution over several orders of magnitude to simple power law we find a slope of -1.0. The sources of FXTs are undoubtedly heterogeneous, the -1 power law is an approximate result of the summation of these multiple sources. Two major contributions come from gamma-ray bursts and stellar flares. Extrapolating from the BATSE catalog of GRBs, we find that the fraction of X-ray flashes that can be the X-ray counterparts of gamma-ray bursts is a function of fluence. Certainly most FXTs are not counterparts of standard gamma-ray bursts. The fraction of FXTs from non-GRB sources, such as magnetic stars, is greatest for the faintest FXTs. Our understanding of the FXT phenomenon remains limited and would greatly benefit from a large, homogeneous data set, which requires a wide-field, sensitive instrument.Comment: 36 pages, 8 figure

XMM-Newton discovery of 217 s pulsations in the brightest persistent supersoft X-ray source in M31

We report on the discovery of a periodic modulation in the bright supersoft X-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newton observations of M31. The source exhibits X-ray pulsations with a period P~217.7 s and a quasi-sinusoidal pulse shape and pulsed fraction ~7-11%. We did not detect statistically significant changes in the pulsation period on the time scale of 4 years. The X-ray spectra of XMMU J004252.5+411540 are extremely soft and can be approximated with an absorbed blackbody of temperature 62-77 eV and a weak power law tail of photon index ~1.7-3.1 in the 0.2-3.0 keV energy band. The X-ray properties of the source and the absence of an optical/UV counterpart brighter than 19 mag suggest that it belongs to M31. The estimated bolometric luminosity of the source varies between ~2e38 and ~8e38 ergs/s at 760 kpc, depending on the choice of spectral model. The X-ray pulsations and supersoft spectrum of XMMU J004252.5+411540 imply that it is almost certainly an accreting white dwarf, steadily burning hydrogen-rich material on its surface. We interpret X-ray pulsations as a signature of the strong magnetic field of the rotating white dwarf. Assuming that the X-ray source is powered by disk accretion, we estimate its surface field strength to be in the range 4e5 G <B_{0}<8e6 G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31 showing coherent pulsations, after the transient supersoft source XMMU J004319.4+411758 with 865.5 s pulsation period.Comment: 11 pages, 4 figures, uses emulateapj style. Submitted to Ap

The discovery of 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 Bo158 in M31. The X-ray flux was modulated by ~83% at a period of 2.78 hr (10017 s) in an XMM-Newton observation taken 2002 Jan 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 Bo158 was stable from observation to observation, with a characteristic cutoff at ~4-6 keV. The photo-electric 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 Bo158 the system is highly compact, with a binary separation of ~10e11 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 a dip-like modulations. We discuss two possible mechanisms explaining the energy-independent modulation observed in Bo 158: i) the obscuration of the central source by highly ionized material that scatters X-rays out of the line of sight; ii) partial covering of an extended source by an opaque absorber which occults varying fractions of the source.Comment: 10 pages, 4 figures, ApJ, submitted, uses emulateapj styl

On the X-ray source luminosity distributions in the bulge and disk of M31: First results from 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 brighter sources which dominate X-ray emission from the bulge. This is the first convincing evidence for 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 et al.(2001) for our Galaxy. This confirms that X-ray population of the disk of M31 is dominated by fainter HMXB sources, while the bulge is populated with brighter LMXBs.Comment: 5 pages, 2 figures, uses emulateapj.sty, Submitted to ApJ

M31 Globular Cluster X-ray Sources: XMM-Newton and Chandra results

(Abridged) We present the results of M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 sq.arcmin of M31. We detected 43 X-ray sources coincident with globular cluster candidates from optical surveys. The estimated isotropic X-ray luminosities of GC sources lie between ~10e35 and ~10e39 erg/s in the 0.3 - 10 keV energy band. The spectral properties and variability of M31 GC X-ray sources are consistent with that derived for the LMXBs in the bulges of M31 and Milky Way. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on a time scales from days to years. 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 and that of the Galactic GC X-ray sources. GC sources make 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 10e37 ergs/s reside in GCs with fraction of GC sources rising to 67-90% for the luminosities above 10e38 ergs/s. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way galaxy, but surprisingly close to that of the early-type galaxies. The brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that globular clusters hosting bright X-ray sources are optically brighter and more metal rich than the rest of M31 globular clusters. The brightest sources with luminosities above ~10e38 ergs/s show tendency to reside in more metal poor clusters.Comment: 17 pages including 7 tables, 17 figures, uses emulateapj style. Submitted to ApJ, a version with full resolution images is available at http://hea.iki.rssi.ru/~tsp/papers/m31_gcs_astro_ph.ps.g

RXTE Observations of an Outburst of Recurrent X-ray Nova GS 1354-644

We present the results of Rossi X-ray Timing Explorer observations of GS 1354-644 during a modest outburst in 1997-1998. The source is one of a handful of black hole X-ray transients that are confirmed to be recurrent in X-rays. A 1987 outburst of the same source observed by Ginga was much brighter, and showed a high/soft spectral state. In contrast the 1997-1998 outburst showed a low/hard spectral state. Both states are typical for black hole binaries. The RXTE All Sky Monitor observed an outburst duration of 150 to 200 days. PCA and HEXTE observations covered ~70 days near the maximum of the light curve and during the flux decline. Throughout the observations, the spectrum can be approximated by Compton upscattering of soft photons by energetic electrons. The hot electron cloud has a temperature kT ~30 keV and optical depth tau~4--5. To fit the data well an additional iron fluorescent line and reflection component are required, which indicates the presence of optically thick cool material, most probably in the outer part of the accretion disk. Dramatic fast variability was observed, and has been analyzed in the context of a shot noise model. The spectrum appeared to be softest at the peaks of the shot-noise variability. The shape of the power spectrum was typical for black hole systems in a low/hard state. We note a qualitative difference in the shape of the dependence of fractional variability on energy, when we compare systems with black holes and with neutron stars. Since it is difficult to discriminate these systems on spectral grounds, at least in their low/hard states, this new difference might be important.Comment: 12 pages, 9 figures, accepted for publication in ApJ (Feb. 2000, v.530), uses emulateapj.st