Measurements of the Cosmic X-ray Background of the Universe and the MVN Experiment

The paper describes previous studies of the cosmic X-ray background (CXB) of the Universe in the energy range 1-100 keV and outline prospects for its investigation with the help of MVN (Monitor Vsego Neba) experiment. The nature of the CXB and its use for studying the cosmological evolution of black holes are briefly discussed. The bulk of the paper is devoted to the methods of CXB measurements, from the first pioneering rocket and balloon-borne experiments to the measurements made with latest-generation orbital X-ray observatories. Particular attention is given to the problems of allowance for the contribution of background events to the measurements with X-ray and hard X-ray instruments.Comment: 20 pages, 17 figures, Published in Astronomy Letter

RXTE observations of X-ray transients IGRJ17091-3624 and IGRJ18539+072

We present results of analysis of observations of two transient sources IGR J17091-3624 and IGR J18539+0727 in April 2003 with RXTE observatory. Obtained energy spectra of sources, and also power spectra of their flux variations give us a possibility to classify them as X-ray binary systems in low/hard spectral state. Parameters of power spectrum of IGR J18539+0727 indicates that this source is a black hole candidate.Comment: 5 pages, accepted for publication in the Astronomy Letter

RXTE observations of Galactic microquasar XTE J1748--288 during its 1998 outburst

We present an analysis of the RXTE observations of the recently discovered Galactic microquasar XTE J1748--288 during its 1998 outburst. The spectral evolution of the source during the outburst can be considered a sequence of qualitatively distinct states. During the first observations, corresponding to the maximum of X-ray flux, the spectrum of the source consisted of a dominating hard power law component and a soft thermal component, which can be described by the model of multicolor disk emission. The hard component contributed >80% to the X-ray luminosity in the 3-25 keV energy band. Overall two-component spectral shape is an attribute of Very High state (VHS) observed previously in BHC, but the domination of hard component is unusual. Later on, as the X-ray source faded, its energy spectrum qualitatively changed, showing High (HS) and then Low (LS) states, both typical for black hole binaries. As the energy spectrum changed, the fast variability also evolved dramatically. Initially the power density spectrum was formed by a dominating band-limited noise component, QPO features at 20-30 Hz and at ~0.5 Hz, and a very low frequency noise component. After a significant decrease of the contribution of the hard spectral component the amplitude of the fractional variability decreased by an order of magnitude and the PDS spectrum adopted a power-law shape with a broad QPO peak around 0.03 Hz. When the system switched to the LS, the PDS shape changed again and the QPOs have not been detected since. When the source was observed in VHS, a clear correlation between QPO parameters and X-ray flux was seen.Comment: 9 pages, 7 figures, accepted for publication in MNRA

Relation between the X-ray and Optical Luminosities in Binary Systems with Accreting Nonmagnetic White Dwarfs

We investigate the relation between the optical (g-band) and X-ray (0.5-10 keV) luminosities of accreting nonmagnetic white dwarfs. According to the present-day counts of the populations of star systems in our Galaxy, these systems have the highest space density among the close binary systems with white dwarfs. We show that the dependence of the optical luminosity of accreting white dwarfs on their X-ray luminosity forms a fairly narrow one-parameter curve. The typical half-width of this curve does not exceed 0.2-0.3 dex in optical and X-ray luminosities, which is essentially consistent with the amplitude of the aperiodic flux variability for these objects. At X-ray luminosities Lx~1e32 erg/sec or lower, the optical g-band luminosity of the accretion flow is shown to be related to its X-ray luminosity by a factor ~2-3. At even lower X-ray luminosities (Lx~1e30 erg/sec), the contribution from the photosphere of the white dwarf begins to dominate in the optical spectrum of the binary system and its optical brightness does not drop below Mg~13-14. Using the latter fact, we show that in current and planned X-ray sky surveys, the family of accreting nonmagnetic white dwarfs can be completely identified to the distance determined by the sensitivity of an optical sky survey in this region. For the Sloan Digital Sky Survey (SDSS) with a limiting sensitivity m_g~22.5, this distance is ~400-600 pcComment: 7 pages, 3 figures, published in Astronomy Letter

IGR J17098-3628: an X-ray Nova discovered by INTEGRAL

We report the discovery with INTEGRAL on March 24, 2005, and follow-up observations of the distant Galactic X-ray nova IGR J17098-3628.Comment: 4 pages, Proceedings of the 6th INTEGRAL Workshop "The Obscured Universe" (July 2-8, 2006, Moscow), ESA SP-62

The ABC of low-frequency quasi-periodic oscillations in Black-Hole Candidates: analogies with Z-sources

Three main types of low-frequency quasi-periodic oscillations (LFQPOs) have been observed in Black Hole Candidates. We re-analyzed RXTE data of the bright systems XTE J1859+226, XTE J1550-564 and GX 339-4, which show all three of them. We review the main properties of these LFQPOs and show that they follow a well-defined correlation in a fractional rms vs. softness diagram. We show that the frequency behavior through this correlation presents clear analogies with that of Horizontal-, Normal- and Flaring-Branch Oscillations in Z sources, with the inverse of the fractional rms being the equivalent of the curvilinear coordinate Sz through the Z track.Comment: 5 pages, 4 figures, accepted by ApJ, uses emulateap

Hard X-ray emission of Sco X-1

We study hard X-ray emission of the brightest accreting neutron star Sco X-1 with INTEGRAL observatory. Up to now INTEGRAL have collected ~4 Msec of deadtime corrected exposure on this source. We show that hard X-ray tail in time average spectrum of Sco X-1 has a power law shape without cutoff up to energies ~200-300 keV. An absence of the high energy cutoff does not agree with the predictions of a model, in which the tail is formed as a result of Comptonization of soft seed photons on bulk motion of matter near the compact object. The amplitude of the tail varies with time with factor more than ten with the faintest tail at the top of the so-called flaring branch of its color-color diagram. We show that the minimal amplitude of the power law tail is recorded when the component, corresponding to the innermost part of optically thick accretion disk, disappears from the emission spectrum. Therefore we show that the presence of the hard X-ray tail may be related with the existence of the inner part of the optically thick disk. We estimate cooling time for these energetic electrons and show that they can not be thermal. We propose that the hard X-ray tail emission originates as a Compton upscattering of soft seed photons on electrons, which might have initial non-thermal distribution.Comment: 9 pages, 7 figures, Accepted for publication in MNRA

Accurate Localization and Identification of Six Hard X-ray Sources from Chandra and XMM-Newton data

We present the results of Chandra and XMM-Newton observations for six hard X-ray sources (IGR J12134-6015, IGR J18293-1213, IGR J18219-1347, IGR J17350-2045, IGR J18048-1455, XTE J1901+014) from the INTEGRAL all-sky survey. Based on these observations, we have improved significantly the localization accuracy of the objects and, therefore, have managed to identify their optical counterparts. Using data from the publicly available 2MASS and UKIDSS infrared sky surveys as well as data from the SOFI/NTT telescope (European Southern Observatory), we have determined the magnitudes of the optical counterparts, estimated their types and (in some cases) the distances to the program objects. A triplet of iron lines with energies of 6.4, 6.7, and 6.9 keV has been detected in the X-ray spectrum of IGR J18048-1455; together with the detection of pulsations with a period of ~1440 s from this source, this has allowed it to be classified as a cataclysmic variable, most likely an intermediate polar. In addition, broadband X-ray spectra of IGR J12134-6015 and IGR J17350-2045 in combination with infrared and radio observations suggest an extragalactic nature of these objects. The source IGR J18219-1347 presumably belongs to the class of high-mass X-ray binaries.Comment: 15 pages, 9 figures. Will be published in Astronomy Letters, 2012, Vol. 38, No. 10, pp. 629-63

Population of persistent high mass X-ray binaries in the Milky Way

We present results of the study of persistent high mass X-ray binaries (HMXBs) in the Milky Way, obtained from the deep INTEGRAL Galactic plane survey. This survey provides us a new insight into the population of high mass X-ray binaries because almost half of the whole sample consists of sources discovered with INTEGRAL. It is demonstrated for the first time that the majority of persistent HMXBs have supergiant companions and their luminosity function steepens somewhere around ~2x10^{36} erg/s. We show that the spatial density distribution of HMXBs correlates well with the star formation rate distribution in the Galaxy. The vertical distribution of HMXBs has a scale-height h~85 pc, that is somewhat larger than the distribution of young stars in the Galaxy. We propose a simple toy model, which adequately describes general properties of HMXBs in which neutron stars accrete a matter from the wind of the its companion (wind-fed NS-HMXBs population). Using the elaborated model we argue that a flaring activity of so-called supergiant fast X-ray transients, the recently recognized sub-sample of HMXBs, is likely related with the magnetic arrest of their accretion.Comment: 17 pages, 12 figures, accepted for publicatiopn in MNRA