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Session 43 - Gamma-ray Burst Counterparts and Afterglows.
Display session, Tuesday, June 09
The energy requirements of gamma ray bursts (GRBs) have in past been poorly constrained because of three major uncertainties: The distances to bursts, the degree of burst beaming, and the efficiency of gamma ray production. The first of these has been effectively resolved by direct evidence from spectroscopy of GRB afterglows.
Afterglows offer at least two tests for beaming, which is the dominant remaining uncertainty (4--6 orders of magnitude) in GRB energy requirements and event rates. First, the bulk Lorentz factor decreases and radiation is beamed into an ever increasing solid angle as the burst remnant expands. It follows that if gamma ray bursts are highly collimated, many more optical and radio transients should be observed without associated gamma rays than with them.
Second, the burst remnant dynamics change qualitatively when the product of the jet opening angle and the bulk Lorentz factor drops below unity, resulting in an observable break in the afterglow light curve. Detailed beaming models yield predictions that conflict with present afterglow light curves. We conclude that gamma ray bursts are unlikely to be highly beamed, and that their energy requirements are near those of isotropic cosmological models.
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