{\em Fermi}/GBM Short Gamma-ray Burst Catalog and Case Study for GRB 170817A/GW 170817

Abstract

Motivating by the discovery of association between GW 170817 and sGRB 170817A, we present a comprehensive analysis for sGRBs observed with Fermi/GBM in 9 operation years and study the properties of sGRB 170817A -like events. We derive a catalog of 275 typical sGRBs and 48 sGRB 170817A-like weak events from the GBM data of 2217 GRBs. We visibly identify two patterns of their light curve, single episode (Pattern I, 61\% of the SGRBs) and multiple episodes (Pattern II, 39\% of the SGRBs). Their duration distribution shows a tentative bimodal feature. Their spectra can be fitted with a cutoff power-law model, except for 4 sGRBs, and the spectral indices normally distribute at $\hat{I}“=0.69\pm 0.40$. Their $E_p$ values show a tentative bimodal distribution with peaks at 145 keV and 500 keV. No correlation among $T_{90}$, $E_p$, and $\hat{I}“$ is found. GRB 170817A is a soft, weak sGRB with $E_p=124\pm 106$ keV, $L_{\mathrm{iso}}=(5.67\pm 4.65)\times 10^{46}\mathrm{erg}{\mathrm{s}}^{-1}$, and $E_{\mathrm{iso}}=(3.23\pm 2.65)\times 10^{46}\mathrm{erg}$. It follows the $E_{\mathrm{iso}}-E_{\mathrm{p}}$ relation of typical short GRBs. Its lightcurve is of Pattern II. Two lightcurve patterns, together with the potential two components in the $E_{\mathrm{p}}$ and $T_{90}$ distributions, we suspect that the current sample may include two distinct types of sGRBs from different progenitors. sGRB 170817A-like events may be from NS-NS mergers and those sGRBs with a Pattern I lightcurve may be from another distinct type of compact binary.

37 pages, 3 tables, 12 figures