The Prompt Gamma‐Ray and Afterglow Energies of Short‐Duration Gamma‐Ray Bursts

I present an analysis of the gamma-ray and afterglow energies of the complete sample of 17 short duration GRBs with prompt X-ray follow-up. I find that 80% of the bursts exhibit a linear correlation between their gamma-ray fluence and the afterglow X-ray flux normalized to t=1 d, a proxy for the kinetic energy of the blast wave ($FX,1~Fgamma^1.01). An even tighter correlation is evident between Egamma,iso and LX,1 for the subset of 13 bursts with measured or constrained redshifts. The remaining 20% of the bursts have values of FX,1/Fgamma that are suppressed by about three orders of magnitude, likely because of low circumburst densities (Nakar 2007). These results have several important implications: (i) The X-ray luminosity is generally a robust proxy for the blast wave kinetic energy, indicating nu_X>nu_c and hence a circumburst density n>0.05 cm⁻³; (ii) most short GRBs have a narrow range of gamma-ray efficiency, with <epsilongamma>~0.85 and a spread of 0.14 dex; and (iii) the isotropic-equivalent energies span 10⁴⁸-10⁵² erg. Furthermore, I find tentative evidence for jet collimation in the two bursts with the highest Egamma,iso, perhaps indicative of the same inverse correlation that leads to a narrow distribution of true energies in long GRBs. 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 outlier fraction of 20% is similar to the proposed fraction of short GRBs from dynamically-formed neutron star binaries in globular clusters. This scenario may naturally explain the bimodality of the FX,1/Fgamma distribution and the low circumburst densities without invoking speculative kick velocities of several hundred km/s.