Rapid-response radio observations of short GRB 181123B with the Australia Telescope Compact Array

G. E. Anderson*, M. E. Bell, J. Stevens, M. D. Aksulu, J. C. A. Miller-Jones, A. J. van der Horst, R. A. M. J. Wijers, A. Rowlinson, A. Bahramian, P. J. Hancock, J-P Macquart, S. D. Ryder, R. M. Plotkin

*Corresponding author for this work

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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 μJy 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 εe = −0.75+0.39−0.40 rather than log εe = −1.13+0.82−1.2 derived without the inclusion of the ATCA values), which is consistent with the range of typical εe derived from GRB afterglow modelling. This allowed us to predict that the forward shock may have peaked in the radio band ∼10 d post-burst, producing detectable radio emission ≳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.

Original languageEnglish
Pages (from-to)4372-4386
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 1 May 2021

Bibliographical note

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 503, Issue 3, May 2021, Pages 4372–4386, https://doi.org/10.1093/mnras/stab727. Copyright 2021 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


  • Gamma-ray burst: individual: GRB 181123B
  • Gamma-ray bursts
  • Neutron star mergers
  • Radio continuum: transients


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