Some Theoretical Implications of Short-Hard Gamma-Ray Burst observations

astro-ph/0703129 [abs, ps, pdf, other] :

Title: Some Theoretical Implications of Short-Hard Gamma-Ray Burst observations


Authors:
Ehud Nakar


Comments: 9 pages, 2 figures; Proceedings of the 36th COSPAR Scientific
Assembly, 2006, Beijing, China. Advances in Space Research in press

Short-hard and long-soft gamma-ray bursts (GRBs) are two distinct phenomena,
but their prompt and afterglow emission show many similarities. This suggests
that two different progenitor systems lead to similar physical processes and
that the prompt and afterglow observations of short-hard GRBs (SHBs) can be
examined using models of long GRBs. Here, I discuss three conclusions that can
be drawn from SHB observations. I show that the lower limit on the Lorentz
factor of SHBs is typically ``only'' 10-50, significantly lower than that of
long GRBs. SHBs with observed X-ray afterglow after 1 day are found to be
roughly as efficient as long GRBs in converting the outflow energy into prompt
gamma-rays. Finally, I examine the origin of SHBs with X-ray dark afterglows
and find that the most plausible explanation is that these SHBs exploded in
extremely low density environment ($n \lesssim 10^{-5} cm^{-3}$)

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