Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-09-28
Astrophys.J.641:L5-L8,2006
Astronomy and Astrophysics
Astrophysics
Revised version, submitted to ApJ
Scientific paper
10.1086/503667
Early X-ray afterglows recently detected by {\it Swift} frequently show a phase of very shallow flux decay lasting from $\sim 10^{2.5} $s up to $\sim 10^4 $s, followed by a steeper, more familiar decay. We suggest that the flat early part of the light curve may be a combination of the decaying tail of the prompt emission and the delayed onset of the afterglow emission observed from viewing angles slightly outside the edge of the region within the jet with prominent afterglow emission, as predicted previously. This would imply that a significant fraction of viewers get very little external shock energy along their line of sight, and, therefore, see a very high $\gamma$-ray to kinetic energy ratio at early times. The early flat phase in the afterglow light curve implies, in a rather robust and model independent manner, a very large $\gamma$-ray efficiency, typically $\gtrsim 90%$, which is very difficult to extract from baryons by internal shocks.
Eichler David
Granot Jonathan
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