Deep ATCA and VLA Radio Observations of Short-GRB Host Galaxies. Constraints on Star Formation Rates, Afterglow Flux, and Kilonova Radio Flares

Abstract

We report the results of an extensive radio-continuum observing campaign of host galaxies of short gamma-ray bursts (GRBs). The goal of this survey was to search for optically obscured star formation, possibly indicative of a population of young short-GRB progenitors. Our sample comprises the hosts and host-galaxy candidates of 16 short-GRBs from 2005 to 2015, corresponding to roughly one-third of the presently known ensemble of well-localized short bursts. Eight GRB fields were observed with ATCA (at 5.5 and 9.0 GHz), and eight fields with the VLA (mostly at 5.5 GHz). The observations typically achieved a 1σ <SUB>rms</SUB> of 5-8 μJy. In most cases, they were performed years after the corresponding burst. No new short-GRB host with optically obscured star formation was found. Only one host galaxy was detected: that of GRB 100206A at z = 0.407. However, its starburst nature was already known from optical/IR data. Its measured radio flux can be interpreted as being due to a star formation rate of about 60 M <SUB>☉</SUB> yr<SUP>-1</SUP>. This is in good agreement with earlier expectations based on the observed broad-band spectral energy distribution of this galaxy. The 15 nondetections constrain the SFRs of the suspected host galaxies and provide upper limits on late-time luminosities of the associated radio afterglows and predicted kilonova radio flares. The nondetection of radio emission from GRB explosion sites confirms the intrinsically low luminosity of short-GRB afterglows and places significant constraints on the parameter space of magnetar-powered radio flares. Luminous radio flares from fiducial massive magnetars have not been found.