Observations of the X-ray Afterglows of GRB011211 and GRB001025 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 fading X-ray object with an average flux in 0.2-10 keV declining from 2.7$\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ during the first 5 ks of 27-ks observation to 1.0$\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ toward the end of the observation. The spectrum of the afterglow can be fit to a power law with alpha=2.16$\pm$0.03 modified for the Galactic absorption. No significant evolution of spectral parameters has been detected during the observation. Similar X-ray spectrum with alpha=2.01$\pm$0.09 has been observed by the XMM from the GRB 001025. The non-detection of any extra absorption in these spectra above the Galactic value is an interesting fact and may impose restrictions to the favorable GRB models involving burst origin in star-forming regions. Finally we discuss soft X-ray lines from GRB 011211 reported by Reeves et al.(2002) and conclude that there is no definitive evidence for the presense of these lines in the spectrum.Comment: submitted to ApJL (7 pages, 4 figures, 2 tables, emulateapj.sty

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

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

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

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

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

Searching for optical transients in real-time : the RAPTOR experiment /.

A rich, but relatively unexplored, region in optical astronomy is the study of transients with durations of less than a day. We describe a wide-field optical monitoring system, RAPTOR, which is designed to identify and make follow-up observations of optical transients in real-time. The system is composed of an array of telescopes that continuously monitor about 1500 square degrees of the sky for transients down to about 12' magnitude in 60 seconds and a central fovea telescope that can reach 16{approx}m' agnitude in 60 seconds. Coupled to the telescope array is a real-time data analysis pipeline that is designed to identify transients on timescales of seconds. In a manner analogous to human vision, the entire array is mounted on a rapidly slewing robotic mount so that the fovea of the array can be rapidly directed at transients identified by the wide-field system. The goal of the project is to develop a ground-based optical system that can reliably identify transients in real-time and ultimately generate alerts with source locations to enable follow-up observations wilh other, larger, telescopes