arXiv subjects

R. A. Sunyaev

Publications and source records attributed to R. A. Sunyaev.

132 records · Page 8Linked to original sources

arXiv1999

GRANAT/SIGMA Observation of the Early Afterglow from GRB 920723 in Soft Gamma-Rays

We present a GRANAT/SIGMA observation of the soft gamma-ray afterglow immediately after GRB 920723. The main burst is very bright. After ~6 s, the burst light curve makes a smooth transition into an afterglow where flux decays as t^{-0.7}. The power-law decay lasts for at least 1000 s; beyond this time, the afterglow emission is lost in the background fluctuations. At least ~20% of main burst energy is emitted in the afterglow. At approximately ~6 s after the trigger, we also observe an abrupt change in the burst spectrum. At t<6 s, the ratio of 8-20 and 75-200 keV fluxes corresponds to the power law spectral index α=0.0-0.3. At t=6 s, the value of αincreases to α= ~1 and stays at this level afterwards. The observed afterglow characteristics are discussed in connection with the relativistic fireball model of gamma-ray bursts.

astro-ph↗

arXiv1999

The X-ray Nova GRS 1739-278 near the Galactic Center

The soft X-ray nova GRS 1739-278 flared up in 1996 near the Galactic center. We present the observations of this interesting source, a black-hole candidate, with the instruments on board the Mir-Kvant module and the RXTE satellite. The TTM data allow the spectrum of the X-ray nova to be studied during the rise in its brightness. The source's spectrum in this period is satisfactorily described by a power law with a gradually increasing index. The RXTE spectra after the source passed its maximum brightness have the appearance that is typical of the high and ultrahigh states of black-hole candidates; they are described by a two-component model. The broad-band spectra of the source are discussed in terms of the processes that proceed near an accreting black hole in a binary system.

astro-ph↗

arXiv1998

The Disc Accretion in Gravitational Field of a Rapidly Rotating Neutron Star with a Rotationally Induced Quadrupole Mass Distribution

We analyze the effect of the quadrupole component in the mass distribution of a rapidly rotating neutron star on energy release in the boundary layer on the surface of the accreting star and in the accretion disk in the cases where the stellar radius is smaller (or larger) than the radius of the marginally stable circular orbit. We calculate the velocities and trajectories of the particles that fall on the stellar surface from the marginally stable orbit for a low- luminosity accreting source. The corresponding external gravitational field of the star is modeled by a new exact solution of the Einstein equations in vacuum. The parameters of this solution are adjusted by reconciling the numerical data for the radius of the marginally stable orbit and the gravitational redshift of Cook et al. (1994) with the corresponding data in the analytical solution. For various equations of state, we consider 1.4 solar mass normal sequences and maximum mass normal sequences.

astro-ph↗

arXiv1998

Hard X-ray observations of the X-ray pulsar A0535+26

We present the results of the observations of the giant bursts from the X-ray pu lsar A0535+26 made by HEXE onboard Mir-Kvant in April 1989, November 1993 and February 1994. The pulse periods were measured, pulse profiles in different energy bands were produced, and their variability was investigated. The power density spectra (PDS) in 2x10^(-3)-1 Hz range is presented, which shape is typical for flicker-noise processes, usually observed in black hole candidates. The noise rms grows with energy from ~20% at 20 keV to ~30% at 80 keV. The source photon spectrum in the 15-200 keV energy range and its variability over the pulse phase are reported. Approximately the shape of the spectrum can be described by the "canonical" model for X-ray pulsars with power-law index g~1.1, cut-off energy E_c~23 keV and folding energy E_f~19 keV. All these parameters are weakly dependent on the luminosity. The most significant deviation from this continuum is observed at ~100 keV in the spectrum of the main pulse maximum. This feature is interpreted as a cyclotron line. Comparison of the HEXE data with the data from BATSE/CGRO (Bildsten et al., 1997) shows that in the high luminosity state (L~10^38 erg/s) the pulsar's pulse profile differs substantially from the pulse profile in the low-luminosity (L~5x10^36 erg/s) state. This difference is explained by the qualitative change of the polar cap structure with formation of the accretion columns.

astro-ph↗

arXiv1998

CMB Radiation in the Direction of a Moving Cluster of Galaxies with Hot Gas: Relativistic Corrections

It has been recently realized (Rephaeli 1995) that the relativistic corrections to the spectral distortions of the cosmic microwave background (CMB) measured in the direction of clusters of galaxies containing hot gas are significant and should be detectable with the forthcoming experiments. In the present paper we calculate the correction terms that are proportional to $V_{r}/c\times kT_e/m_ec^2$ and $(V/c)^2$ to the standard formulae describing the spectral distortions caused by the bulk motion of the free electrons (kinematic effect) and due to the presence of the hot gas (thermal effect) for the case of a cluster having a peculiar velocity $V$ ($V_{r}$ is its radial component). The results of our analytical calculations are confirmed by Monte-Carlo simulations (Sazonov & Sunyaev 1998).

astro-ph↗

arXiv1995

X-ray Spectrum of the Black Hole Candidate X1755-338

We report the first detection of a hard power-law tail in the X-ray spectrum of the black hole candidate (BHC) binary X1755-338, which was observed in 1989 March-September during the TTM Galactic Centre survey. In addition, an ultrasoft thermal component with a temperature of ~1.1-1.4 keV was also detected. We demonstrate that the soft and hard X-ray components of X1755-338 vary independently, as in the spectra of the well known BHCs LMC X-1, LMC X-3 and GX339-4 in their high (intensity) state. If the hard tail observed from X1755-338 is generated near the black hole by energetic electrons up-scattering low energy photons, the un-correlated variations imply that the soft X-rays from accretion disc may not be the main photon seeds needed for inverse Comptonization. The TTM observations strongly suggest that X1755-338 does indeed belong to the family of BHCs.

astro-ph↗