Free-fall accretion and emitting caustics in wind-fed X-ray sources

In wind-fed X-ray binaries the accreting matter is Compton cooled and falls freely onto the compact object. The matter has a modest angular momentum $l$ and accretion is quasi-spherical at large distances from the compact object. Initially small non-radial velocities grow in the converging supersonic flow and become substantial in the vicinity of the accretor. The streamlines with $l>(GMR_*)^{1/2}$ (where $M$ and $R_*$ are the mass and radius of the compact object) intersect outside $R_*$ and form a two-dimensional caustic which emits X-rays. The streamlines with low angular momentum, $l<(GMR_*)^{1/2}$, run into the accretor. If the accretor is a neutron star, a large X-ray luminosity results. We show that the distribution of accretion rate/luminosity over the star surface is sensitive to the angular momentum distribution of the accreting matter. The apparent luminosity depends on the side from which the star is observed and can change periodically with the orbital phase of the binary. The accretor then appears as a `Moon-like' X-ray source.Comment: 8 pages, accepted to MNRA

On the Efficiency of Internal Shocks in Gamma-Ray Bursts

The fraction of a fireball kinetic kinetic energy that can be radiated away by internal shocks is sensitive to the amplitude of initial fluctuations in the fireball. We give a simple analytical description for dissipation of modest-amplitude fluctuations and confirm it with direct numerical simulations. At high amplitudes, the dissipation occurs in a non-linear regime with efficiency approaching 100 %. Most of the explosion energy is then radiated away by the prompt GRB and only a fraction remains to be radiated by the afterglow.Comment: submitted to ApJ Letter

Super-Eddington accretion disc around a Kerr black hole

We calculate the structure of accretion disc around a rapidly rotating black hole with a super-Eddington accretion rate. The luminosity and height of the disc are reduced by the advection effect. In the case of a large viscosity parameter, alpha > 0.03, the accretion flow strongly deviates from thermodynamic equilibrium and overheats in the central region. With increasing accretion rate, the flow temperature steeply increases, reaches a maximum, and then falls off. The maximum is achieved in the advection dominated regime of accretion. The maximum temperature in the disc around a massive black hole, M=10^8 M_sun, with alpha=0.3 is of order 3 x 10^8 K. Discs with large accretion rates can emit X-rays in quasars as well as in galactic black hole candidates.Comment: accepted for publication in MNRAS, 8 page

Early Stages of the GRB Explosion

Physics of GRB blast waves is discussed with a focus on two effects: (1) pair creation in the external medium by the gamma-ray front and (2) decay of neutrons ahead of the decelerating blast wave. Both effects impact the afterglow mechanism at radii up to 10^{17}cm.Comment: 5 pages, to appear in the proceedings of the 2003 GRB Conference, Santa Fe, Sep 8-1