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 hr post-burst, when the source had risen above the horizon. While no radio afterglow was detected during the 8.3 hr observation, we obtained force-fitted flux densities of $7 \pm 12$ and $15 \pm 11~\mu$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$\epsilon_e = -0.75^{+0.39}_{-0.40}$ rather than log$\epsilon_e = -1.13^{+0.82}_{-1.2}$ 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 $\sim10$ days post-burst, producing detectable radio emission $\gtrsim3-4$ days 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.Comment: 15 pages, 7 figures, accepted for publication in MNRA

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 hr post-burst, when the source had risen above the horizon. While no radio afterglow was detected during the 8.3 hr observation, we obtained force-fitted flux densities of $7 \pm 12$ and $15 \pm 11~\mu$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$\epsilon_e = -0.75^{+0.39}_{-0.40}$ rather than log$\epsilon_e = -1.13^{+0.82}_{-1.2}$ 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 $\sim10$ days post-burst, producing detectable radio emission $\gtrsim3-4$ days 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

Exploring the GRB population:robust afterglow modelling

Gamma-ray bursts (GRBs) are ultra-relativistic collimated outflows, which emit synchrotron radiation throughout the entire electromagnetic spectrum when they interact with their environment. This afterglow emission enables us to probe the dynamics of relativistic blast waves, the microphysics of shock acceleration, and environments of GRBs. We perform Bayesian inference on a sample of GRB afterglow data sets consisting of 22 long GRBs and 4 short GRBs, using the afterglow model "scalefit", which is based on 2D relativistic hydrodynamic simulations. We make use of Gaussian processes to account for systematic deviations in the data sets, which allows us to obtain robust estimates for the model parameters. We present the inferred parameters for the sample of GRBs, and make comparisons between short GRBs and long GRBs in constant-density and stellar-wind-like environments. We find that in almost all respects such as energy and opening angle, short and long GRBs are statistically the same. Short GRBs however have a markedly lower prompt $\gamma$-ray emission efficiency than long GRBs. We also find that for long GRBs in ISM-like ambient media there is a significant anti-correlation between the fraction of thermal energy in the magnetic fields, $\epsilon_B$, and the beaming corrected kinetic energy. Furthermore, we find no evidence that the mass-loss rates of the progenitor stars are lower than those of typical Wolf-Rayet stars.Comment: Submitted to MNRAS. Accepted 2022 January 17. Received 2021 December 29; in original form 2021 June 2

GRB 200411A: ATCA 5/9 GHz radio observations

We used the Australia Telescope Compact Array (ATCA) to perform a radio observation of the short GRB 200411A

GRB 200219A: ATCA 5/9 GHz radio observations

We used the Australia Telescope Compact Array (ATCA) to perform a radio observation of the short GRB 200219A. The scheduled observations began on 2020 Feb 22.8 UT for 7.5 hours (3.5 days post-burst; Fermi GBM Team GCN 27123, A. Y. Lien et al., GCN 27125). No radio source was detected at the XRT position (J.P. Osborne et al., GCN 27138), with preliminary 3 sigma upper-limits of 53 microJy and 57 microJy at 5.5 and 9 GHz, respectivel

GRB 200219A: ATCA 5/9 GHz radio observations

We used the Australia Telescope Compact Array (ATCA) to perform a radio observation of the short GRB 200219A. The scheduled observations began on 2020 Feb 22.8 UT for 7.5 hours (3.5 days post-burst; Fermi GBM Team GCN 27123, A. Y. Lien et al., GCN 27125). No radio source was detected at the XRT position (J.P. Osborne et al., GCN 27138), with preliminary 3 sigma upper-limits of 53 microJy and 57 microJy at 5.5 and 9 GHz, respectivel