Abstract. Using Fourier frequency resolved X-ray spectroscopy we study short term spectral variability in luminous LMXBs. With RXTE/PCA observations of 4U1608-52 and GX 340+0 on the horizontal/normal branch of the color-intensity diagram we show that aperiodic and quasiperiodic variability on ∼second-millisecond time scales is caused primarily by variations of the luminosity of the boundary layer. The emission of the accretion disk is less variable on these time scales and its power density spectrum follows P disk ( f ) ∝ f −1 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, i.e. independent of the nature of the "clock", the actual luminosity modulation takes place on the neutron star surface. The boundary layer spectrum remains nearly constant in the course of the luminosity variations and is represented to certain accuracy by the Fourier frequency resolved spectrum. In the considered rangeṀ ∼ (0.1-1)Ṁ Edd it depends weakly on the global mass accretion rate and in the limitṀ ∼Ṁ Edd is close to Wien spectrum with kT ∼ 2.4 keV (in the distant observer's frame). The spectrum of the accretion disk emission is significantly softer and in the 3-20 keV range is reasonably well described by a relativistic disk model with a mass accretion rate consistent with the value inferred from the observed X-ray flux.

Aims. We study the spectrum of the cosmic X-ray background (CXB) in energy range ∼5−100 keV. Methods. Early in 2006 the INTEGRAL observatory performed a series of four 30 ks observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background (CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. Results. The spectrum of the cosmic X-ray background in the energy band 5−100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is ∼10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20−50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CXB origin.

Since its launch INTEGRAL has devoted a significant fraction of its observing time to the Galactic plane region. We present the results of INTEGRAL observations of the inner spiral arms of the Galaxy (Norma, Scutum and Sagittarius) with attention to high mass X-ray binaries. The increased sensitivity of the survey and possibility to detect absorbed sources significantly enlarged our sample of HMXBs in comparison with previous studies. For some absorbed sources we present for the first time high quality broadband (1-100 keV) energy spectra. We confirm pulsations of the X-ray flux of IGR J16358-4726, discovered by CHANDRA, and report discovery of pulsations with a period of 228 ± 6 s from IGR J16465-4507. We confirm that the Galactic high mass X-ray binary distribution is significantly different from the distribution of low mass X-ray binaries concentrated in the Galactic Center. A large fraction of detected high mass X-ray binaries are absorbed sources.

Abstract. Results of simultaneous INTEGRAL and optical observations of the galactic microquasar SS433 in May 2003and INTEGRAL /RXTE observations in March 2004 are presented. Persistent precessional variability with a maximum to minimum uneclipsed hard X-ray flux ratio of ∼4 is discovered. The 18-60 keV X-ray eclipse is found to be in phase with optical and near infrared eclipses. The orbital eclipse observed by INTEGRAL in May 2003 is at least two times deeper and apparently wider than in the soft X-ray band. The broadband 2-100 keV X-ray spectrum simultaneously detected by RXTE/INTEGRAL in March 2004 can be explained by bremsstrahlung emission from optically thin thermal plasma with kT ∼ 30 keV. Optical spectroscopy with the 6-m SAO BTA telescope confirmed the optical companion to be an A5-A7 supergiant. For the first time, spectorscopic indications of a strong heating effect in the optical star atmosphere are found. The measurements of absorption lines which are presumably formed on the non-illuminated side of the supergiant yield its radial velocity semi-amplitude K v = 132 ± 9 km s −1 . The analysis of the observed hard X-ray light curve and the eclipse duration, combined with the spectroscopically determined optical star radial velocity corrected for the strong heating effect, allows us to model SS433 as a massive X-ray binary. Assuming that the hard X-ray source in SS433 is eclipsed by the donor star that exactly fills its Roche lobe, the masses of the optical and compact components in SS433 are suggested to be M v ≈ 30 M and M x ≈ 9 M , respectively. This provides further evidence that SS433 is a massive binary system with supercritical accretion onto a black hole.