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

Boundary layer emission in luminous LMXBs

We show that aperiodic and quasiperiodic variability of bright LMXBs - atoll and Z- sources, on ~sec - msec time scales is caused primarily by variations of the boundary layer luminosity. The accretion disk emission is less variable on these time scales and its power density follows 1/f law, contributing to observed flux variation at low frequencies and low energies only. The kHz QPOs have the same origin as variability at lower frequencies - independent of the nature of the "clock", the actual luminosity modulation takes place on the NS surface. The boundary layer spectrum remains nearly constant during luminosity variations and can be represented by the Fourier frequency resolved spectrum. In the range of Mdot~(0.1-1)*Mdot_Edd it depends weakly on the global mass accretion rate and in the limit Mdot~Mdot_Edd is close to Wien spectrum with kT~2.4 keV. Its independence on the Mdot lends support to the suggestion by Inogamov & Sunyaev (1999) that the boundary layer is radiation pressure supported. Based on the knowledge of the boundary layer spectrum we attempt to relate the motion along the Z-track to changes of physically meaningful parameters. Our results suggest that the contribution of the boundary layer to the observed emission decreases along the Z-track from conventional ~50% on the horizontal branch to a rather small number on the normal branch. This decrease can be caused, for example, by obscuration of the boundary layer by the geometrically thickened accretion disk at Mdot~Mdot_Edd. Alternatively, this can indicate significant change of the structure of the accretion flow at Mdot~Mdot_Edd and disappearance of the boundary layer as a distinct region of the significant energy release associated with the NS surface.Comment: Astronomische Nachrichten, 326, No.9, p.812 (2005

Luminosity function of faint Galactic sources in the Chandra bulge field

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 L(2-10 keV)~1e30 erg/sec 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 1e30-1e32 erg/sec per unit stellar mass is L(2-10 keV)/M*=(1.7+/-0.3)e27 erg/sec/Msun. Taking into account sources in the luminosity range 1e32-1e34 erg/sec from Sazonov et al. (2006), the cumulative luminosity density in the broad luminosity range 1e30-1e34 erg/sec becomes L(2-10 keV)/M*=(2.4+/-0.4)e27 erg/sec/Msun. 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.Comment: 5 pages, 4 figures, Accepted for publication in MNRA

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

Using the 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 msec. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behavior differs notably from 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 sec. 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 disk, in the hard spectral state is larger by a factor of >5-10 than in the soft spectral state. Modeling the transfer function of the disk, we estimate the inner radius of the accretion disk R_in~100R_g in the hard and R_in<10R_g in the soft state for a 10M_sun black hole.Comment: submitted to MNRA

Soft state of Cygnus X-1: stable disk and unstable corona

Two component X-ray spectra (soft multicolor black body plus harder power law) are frequently observed from accreting black holes. These components are presumably associated with the different parts of the accretion flow (optically thick and optically thin respectively) in the vicinity of the compact source. Most of the aperiodic variability of the X-ray flux on the short time scales is associated with the harder component. We suggest that drastically different amplitudes of variability of these two components are simply related to the very different viscous time scales in the geometrically thin and geometrically thick parts of the accretion flow. In the geometrically thin disks variations of viscosity or mass accretion rate occurring at large radius from the black hole on the local dynamical or thermal time scales do not cause any significant variations of the mass accretion rate at smaller radii due to a very long diffusion time. Any variations on the time scales shorter than the diffusion time scale are effectively dampened. On the contrary such variations can easily survive in the geometrically thick flows and as a result the mass accretion rate in the innermost region of the flow will reflect modulations of the mass accretion rate added to the flow at any distance from the black hole. Therefore if primary instabilities operate on the short time scales then the stability of the soft component (originating from the geometrically thin and optically thick flow) and variability of the hard component (coming from the geometrically thick and optically thin flow) are naturally explained.Comment: 8 pages; accepted for publication in MNRAS; replaced with accepted versio