Rapid-response Radio Observations of Short GRB 181123B with the Australia Telescope Compact Array

We introduce the Australia Telescope Compact Array (ATCA) rapid-response mode by presenting the first successful trigger on the short-duration gamma-ray burst (GRB) 181123B. Early-time radio observations of short GRBs may provide vital insights into the radio afterglow properties of Advanced LIGO- and Virgo-detected gravitational wave events, which will in turn inform follow-up strategies to search for counterparts within their large positional uncertainties. The ATCA was on target within 12.6 h post-burst, when the source had risen above the horizon. While no radio afterglow was detected during the 8.3 h observation, we obtained force-fitted flux densities of 7 +/- 12 and 15 +/- 11 mu Jy at 5.5 and 9 GHz, respectively. Afterglow modelling of GRB 181123B showed that the addition of the ATCA force-fitted radio flux densities to the Swift X-ray Telescope detections provided more stringent constraints on the fraction of thermal energy in the electrons (log epsilon(e) = -0.75(-0.40)(+0.39) rather than log epsilon(e) = -1.13(-1.2)(+0.82) derived without the inclusion of the ATCA values), which is consistent with the range of typical epsilon(e) derived from GRB afterglow modelling. This allowed us to predict that the forward shock may have peaked in the radio band similar to 10 d post-burst, producing detectable radio emission greater than or similar to 3-4 d post-burst. Overall, we demonstrate the potential for extremely rapid radio follow-up of transients and the importance of triggered radio observations for constraining GRB blast wave properties, regardless of whether there is a detection, via the inclusion of force-fitted radio flux densities in afterglow modelling efforts.