HEAD Division Meeting 1999, April 1999
Session 1. Gamma Ray Bursts
Oral, Monday, April 12, 1999, 8:30am-10:04am, Colonial Room

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[1.05] Constraints on Beaming in Gamma Ray Bursts

C.D. Dermer (NRL), K.E. Mitman (TJHSST, NRL), J. Chiang (JILA)

One of the fundamental uncertainties in studies of gamma-ray bursts (GRBs) is the fraction of the full sky into which the burst emission is beamed. The amount of beaming determines the total burst energy, the number density of GRB sources, and the nature of the sources which produce GRBs. Here we present detailed numerical simulations of emission from blast waves which radiate nonthermal synchrotron and synchrotron self-Compton emission by sweeping up matter from the surrounding medium. Breaks in the temporal afterglow behavior are produced by collimated blast waves as they decelerate. By comparing with observations of power-law afterglow behavior, we constrain the maximum degree of beaming. Highly collimated (<< 1% models are ruled out. We calculate the spectra of misaligned GRBs and compare with studies of X-ray transients in order to place additional limits on beaming.

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