Session B16 - Neutrino and Gamma Ray Astrophysics.
MIXED session, Saturday morning, April 29
202C, Convention Center
The identification of flaring and fading X-ray, optical and radio counterparts to gamma-ray burst (GRB) sources, and the large energy releases implied by redshift measurements, find a consistent explanation in an expanding relativistic blast-wave model. This model has been called into question by observations of spectra from GRBs that are harder than can be produced through the synchrotron process. If GRBs originate from the collapse of massive stars, then circumstellar clouds near burst sources will be illuminated by intense gamma radiation, and the electrons in these clouds will be rapidly scattered to energies as large as several hundred keV. Low-energy photons that subsequently pass through the hot plasma will be scattered to higher energies, and this effect resolves the line-of-death objection to the synchrotron shock model. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electron-positron pairs that can be detected within ~ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy would reveal past GRB explosions.