The Prompt Gamma-Ray and Afterglow Energies of Short-Duration Gamma-Ray Bursts
Abstract
I present an analysis of the γ-ray and afterglow energies of a complete sample of 16 short-duration GRBs with prompt X-ray follow-up. I find that 80% of the bursts exhibit a linear correlation between their γ-ray fluence and the afterglow X-ray flux normalized to t=1 day, a proxy for the kinetic energy of the blast wave (FX,1~F1.01+/-0.27γ). An even tighter correlation is evident between the isotropic γ-ray energy, Eγ,iso, and the X-ray luminosity at t=1 day, LX,1, for the subset of 12 bursts with measured or constrained redshifts. The remaining 20% of the bursts have values of FX,1/Fγ that are suppressed by about 3 orders of magnitude, likely as a result of low circumburst densities; this has been noted based on a smaller sample by Nakar (2007). These results have several important implications: (1) the X-ray luminosity is generally a robust proxy for the blast wave kinetic energy, indicating νX>νc, and hence a circumburst density n>~0.05 cm-3 (2) most short GRBs have a narrow range of γ-ray efficiency, with <ɛγ>~0.1 and a spread of 0.3 dex; and (3) the isotropic-equivalent energies span 1048-1053 erg. Furthermore, I find tentative evidence for jet collimation (opening angle, θj~6deg) in GRB 061006 leading to Eγ~4×1048 erg, similar to other short bursts with jet breaks. I find no clear evidence for a relation between the overall energy release and host galaxy type, but a positive correlation with duration may be present, albeit with a large scatter. Finally, I note that the low-density hypothesis for the outliers can be explained in the context of neutron star-neutron star (NS-NS) mergers in globular clusters (as opposed to large kick velocities), but present short GRB rate estimates may be an order of magnitude too large for this scenario to work.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2007
- DOI:
- 10.1086/522195
- arXiv:
- arXiv:astro-ph/0702694
- Bibcode:
- 2007ApJ...670.1254B
- Keywords:
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- Gamma Rays: Bursts;
- Astrophysics
- E-Print:
- Submitted to ApJ