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

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

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