Classification of Photospheric Emission in Short GRBs

In order to better understand the physical origin of short-duration gamma-ray bursts (GRBs), we perform a time-resolved spectral analysis on a sample of 70 pulses in 68 short GRBs with burst durations T 90 ≲ 2 s detected by the Fermi/Gamma-ray Burst Monitor. We apply a Bayesian analysis to all spectra that have statistical significance S ≥ 15 within each pulse and apply a cutoff power-law model. We then select in each pulse the time bin that has the maximum value of the low-energy spectral index for further analysis. Under the assumption that the main emission mechanism is the same throughout each pulse, this analysis is indicative of pulse emission. We find that about 1/3 of the short GRBs are consistent with a pure, nondissipative photospheric model, at least around the peak of the pulse. This fraction is larger than the corresponding fraction (1/4) obtained for long GRBs. For these bursts, we find (i) a bimodal distribution in the values of the Lorentz factors and the hardness ratios and (ii) an anticorrelation between T 90 and the peak energy, E pk:. This correlation disappears when we consider the entire sample. Our results thus imply that the short GRB population may in fact be composed of two separate populations: one that is a continuation of the long GRB population to shorter durations, and another that is distinctly separate with different physical properties. Furthermore, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron).