Amotz Shemi

The appearance of cosmic fireballs

The influence of matter on a radiation fireball is examined. Even when a small amount of baryonic material is present, most of the radiation energy of the fireball will be converted to a kinetic energy of the matter, and almost no electromagnetic signal will be observed. The implications of this result for several processes in which such a fireball might occur are discussed. 10 refs.

Hydrodynamics of relativistic fireballs

Many models of gamma-ray bursts involve a fireball, which is an optically thick concentration of radiation energy with a high ratio of energy density to rest mass. We examine analytically and numerically the evolution of a relativistic fireball. We show that, after an early rearrangement phase, most of the matter and energy in the fireball is concentrated within a narrow shell. The shell propagates at nearly the speed of light, with a frozen radial profile, and according to a simple set of scaling laws. The spectrum of the escaping radiation is harder at early times and softer later on. Depending on the initial energy-to-mass ratio, the final outcome of a fireball is either photons with roughly the initial temperature or ultrarelativistic baryons. In the latter case, the energy could be converted back to gamma-rays via interaction with surrounding material.

Fireballs in the Galactic halo and gamma-ray bursts

If gamma ray burst sources are in the galactic halo they inevitably involve a creation of an opaque pair plasma fireball, just like in cosmological sources. We find that the typical physical conditions in a galactic halo fireball are: optical depth

Gamma-ray bursts from interaction of relativistic flows with radiation fields

\ap 10^8

Neutron star mergers and γ‐ray bursts

, thermal energy

TAUVEX UV imager on the SRG

\ap 100 KeV

Cosmic rays from accreting isolated neutron stars

, maximal relativstic expansion

Are unidentified extreme-ultraviolet sources the closest neutron stars?

\G \ap 300

Cosmic fireballs and gamma ray bursts

and a maximal baryonic load of

GRBs from Compton drag of relativistic flows

\ap 10^{-15} M_\odot