Physical Characteristics of Short GRB from Afterglow Observational Light Curves

With the advent of the multi-messenger era of astronomy, that is the combined electromagnetic (EM)/gravitational-wave (GW) observations and studies of neutron stars and black holes, the interest in short gamma-ray bursts (SGRBs) has increased tremendously. Indeed, until the historic Aug. 17, 2017 simultaneous detection of GWs and EM radiation (from gamma rays to radio waves) produced by the merger of two neutron stars (NS), the GW-GRB 170817 event, the hypothesis that SGRBs (bursts with durations less than ~ 2.0 seconds) are produced by the merging of two neutron stars, was widely but not unanimously held. And now similar events (or at least candidates of NS-NS or NS-BH mergers, are coming faster and faster. Observations of afterglows of GRBs in general, and SGRBs in particular (as these are accompanied by a kilonova explosion), have also increased manifold, particularly in the red and near-infrared bands, which are most interesting for kilonova detections. Despite the fact that SGRBs represent only about 10 % of all GRBs, one can find numerous reports of observations (positive or negative) of afterglows on the Gamma-ray Coordinates Network (GCN). Analysis and fitting for the light curves that can be constructed from those observations can then yield valuable information on the physical characteristics of such bursts/mergers. Indeed, Fong et al. (2015) collected and analyzed afterglow observations of SGRBs from 2005 to 2014, covering their broadband emission from X-rays to radio. Using the widely adopted theoretical prescription of Panaitescu & Kumar (2000) for calculating afterglow fluxes, Fong et al. (2015) extracted individual and statistical values for the bursts' and afterglows' physical parameters, i.e. kinetic energy, medium density, electron and magnetic energy fractions, and jet opening angles. We have extended that work to the last 5 years (Jan. 2015 to Dec. 2019), using Swift events (XX short bursts recorded during that period) and Fermi events (XX events), aiming to fit or constrain the afterglow light curves with physical parameters, to the extent possible, and to try to identify cases that most resemble those that have had a kilonova detection. We present the most interesting cases that we have uncovered, along with statistical results on the physical parameters that determine the fits to the light curves.