E. Churazov

A hard X-ray survey of the Galactic-Center region with the IBIS Telescope of the INTEGRAL Observatory : A catalog of sources

From August 23 through September 24, 2003, the INTEGRAL Observatory conducted a deep survey of the Galactic-Center region with a record-breaking sensitivity at energies above 20 keV. We have analyzed the images of the Galactic-Center region obtained with the ISGRI detector of the IBIS telescope (15–200 keV) and give a catalog of detected sources. We detected a total of 60 sources with fluxes above 1.5 mCrab, 44 and 3 of which were previously identified as binary systems of different classes in our Galaxy and as extragalactic objects, respectively. We discovered one new source.During the period Aug.23-Sept.24 2003, the INTEGRAL observatory performed an ultra deep survey of the Galactic Center region with a record sensitivity at energies higher than 20 keV. We have analized images of the Galactic Center region obtained with the ISGRI detector of the IBIS telescope (15-200 keV) and present here a catalog of detected sources. In total, 60 sources with a flux higher than 1.5 mCrab have been detected. 44 of them were earlier identified as Galactic binary systems, 3 are extragalactic objects. 2 new sources are discovered.During the period Aug.23-Sept.24 2003, the INTEGRAL observatory performed an ultra deep survey of the Galactic Center region with a record sensitivity at energies higher than 20 keV. We have analized images of the Galactic Center region obtained with the ISGRI detector of the IBIS telescope (15-200 keV) and present here a catalog of detected sources. In total, 60 sources with a flux higher than 1.5 mCrab have been detected. 44 of them were earlier identified as Galactic binary systems, 3 are extragalactic objects. 2 new sources are discovered.

Frequency-resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state

ABSTRA C T Using RXTE/PCA data, we study the fast variability of the reflected emission in the soft spectral state of Cyg X-1 by means of Fourier frequency-resolved spectroscopy. We find that the rms amplitude of variations of the reflected emission has the same frequency dependence as the primary radiation down to time-scales of &30‐50 ms. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behaviour differs notably from that of the hard spectral state, in which variations of the reflected flux are significantly suppressed in comparison with the primary emission, on time-scales shorter than ,0.5‐1 s. If related to the finite light-crossing time of the reflector, these results suggest that the characteristic size of the reflector, presumably an optically thick accretion disc, in the hard spectral state is larger by a factor of *5‐10 than in the soft spectral state. Modelling the transfer function of the disc, we estimate the inner radius of the accretion disc to be Rin , 100Rg in the hard state and Rin & 10Rg in the soft state for a 10-M( black hole.

New class of low frequency QPOs: Signature of nuclear burning or accretion disk instabilities?

We report the discovery of a new class of low frequency quasi-periodic variations of the X-ray flux in the X-ray bursters 4U1608-52 and 4U1636-536. We also report an occasional detection of a similar QPO in Aql X-1. The QPOs, associated with flux variations at the level of percents, are observed at a frequency of 7-9

Reflection and noise in the low spectral state of GX 339{4

\times 10^{-3}

Luminosity function of faint Galactic sources in the Chandra bulge field

Hz. While usually the relative amplitude of flux variations increases with energy, the newly discovered QPOs are limited to the softest energies (1-5 keV). The observations of 4U1608-52 suggest that these QPOs are present only when the source X-ray luminosity is within a rather narrow range and they disappear after X-ray bursts. Approximately at the same level of the source luminosity, type I X-ray bursts cease to exist. Judging from this complex of properties, we speculate that a special mode of nuclear burning at the neutron star surface is responsible for the observed flux variations. Alternatively, some instabilities in the accretion disk may be responsible for these QPOs.

Properties of the hard X-ray radiation from Cygnus X-l and 1E1740.7 — 2942

We analyze RXTE/PCA observations of GX 339{4 in the low spectral state from 1996{1997 and show that the pattern of its spectral and temporal variability is nearly identical to that of Cyg X-1. In particular, a tight correlation exists between the QPO centroid frequency and the spectral parameters. An increase of the QPO centroid frequency is accompanied with an increase of the amplitude of the reflected component and a steepening the slope of the underlying power law. Fourier frequency resolved spectral analysis showed, that the variability of the reflected component at frequencies higher than 1 10 Hz is suppressed in comparison with that of the primary emission.

LMXBs and black hole candidates in the Galactic Center region

We study the statistical properties of faint X-ray sources detected in the Chandra Bulge Field. The unprecedented sensitivity of the Chandra observations allows us to probe the population of faint Galactic X-ray sources down to luminosities L2 10keV 10 30 erg s 1 at the Galactic Center distance. We show that the luminosity function of these CBF sources agrees well with the luminosity function of sources in the Solar vicinity (Sazonov et al. 2006). The cumulative luminosity density of sources detected in the CBF in the luminosity range 10 30 10 32 erg s 1 per unit stellar mass is L2 10keV=M? = (1:7 0:3) 10 27 erg s 1 M 1 . Taking into account sources in the luminosity range 10 32 10 34 erg s 1 from Sazonov et al. (2006), the cumulative luminosity density in the broad luminosity range 10 30 10 34 erg s 1 becomes L2 10keV=M? = (2:4 0:4) 10 27 erg s 1 per M . The majority of sources with the faintest luminosities should be active binary stars with hot coronae based on the available luminosity function of X-ray sources in the Solar environment.

Search for outbursts in the narrow 511-keV line from compact sources based on INTEGRAL data

The entire dataset of the GRANAT/SIGMA observations of Cyg X-1 and 1E1740.7-2942 in 1990-1994 was analyzed in order to search for correlations between primary observational characteristics of the hard X-ray (40-200 keV) emission - hard X-ray luminosity, hardness of the spectrum (quantified in terms of the best-fit thermal bremsstrahlung temperature kT) and the RMS of short-term flux variations. Although no strict point-to-point correlations were detected certain general tendencies are evident. It was found that for Cyg X-1 the spectral hardness is in general positively correlated with relative amplitude of short-term variability. The correlation of similar kind was found for X-ray transient GRO J0422+32 (X-ray Nova Persei 1992) and recently for GX339-4. For both sources approximate correlation between kT and L_X was found. At low hard X-ray luminosity - below 10E37 erg/sec - the kT increases with increase of L_X. At higher luminosity the spectral hardness depends weaker or does not depend at all on the hard X-ray luminosity. The low luminosity end of these approximate correlations (low kT and low RMS) corresponds to extended episodes of very low hard X-ray flux occurred for both sources during SIGMA observations.

GRANAT/SIGMA observations of X-ray Nova Persei 1992

Abstract The central part of the Galactic Plane has been intensively observed by GRANAT/SIGMA in 1990–1994. At least 11 sources are seen on the averaged 35–100 keV image of this region, most of them being low mass X-ray binaries. With GRANAT/SIGMA observations the X-ray bursters are recognized as sources emitting hard X-rays (up to ∼ 100 keV) during substantial fraction of time. This “bimodal” behavior (soft and hard spectral states) of X-ray bursters resembles that of black hole candidates (BHC), although unlike BHCs soft state for the bursters seems to be the dominant one. On average the spectra of bursters in the SIGMA energy band (above 35 keV) were found to be steeper than that of BHCs. This difference is discussed in terms of Comptonization model.

Long-Term Variability of the Hard X-ray Source GRS 1758-258

We present the results of a systematic search for outbursts in the narrow positron annihilation line on various time scales (5 × 104–106 s) based on the SPI/INTEGRAL data obtained from 2003 to 2008. We show that no outbursts were detected with a statistical significance higher than ∼6σ for any of the time scales considered over the entire period of observations. We also show that, given the large number of independent trials, all of the observed spikes could be associated with purely statistical flux fluctuations and, in part, with a small systematic prediction error of the telescope’s instrumental background. Based on the exposure achieved in ∼6 yr of INTEGRAL operation, we provide conservative upper limits on the rate of outbursts with a given duration and flux in different parts of the sky.