The Radio Afterglow of the Ultralong GRB 220627A

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Abstract
We present the discovery of the radio afterglow of the most distant ultralong gamma-ray burst (GRB) detected to date, GRB 220627A at redshift z = 3.084. Its prompt gamma-ray light curve shows a double-pulse profile, with the pulses separated by a period of quiescence lasting ∼15 minutes, leading to early speculation it could be a strongly gravitationally lensed GRB. However, our analysis of the Fermi Gamma-ray Burst Monitor spectra taken during the time intervals of both pulses show clear differences in their spectral energy distributions, disfavouring the lensing scenario. We observed the radio afterglow from 7 to 456 days postburst: an initial, steep decay ( F ν ∝ t −2 ) is followed by a shallower decline ( F ν ∝ t −1/2 ) after ∼20 days. There are three scenarios that could explain these radio properties: (i) energy injection from an additional, slower ejecta component catching up to the external shock; (ii) a stratified density profile going as n ∝ r −8/3 ; or alternatively, (iii) the presence of a slow, wide ejecta component in addition to a fast, narrow ejecta component. We also conducted an independent test of the lensing hypothesis via very long baseline interferometry (VLBI) observations at ∼12 days postburst by searching, for the first time, for multiple images of the candidate lensed GRB afterglow. Our experiment highlighted the growing need for developments in real-time correlation capabilities for time-critical VLBI experiments, particularly as we advance towards the SKA and ngVLA era of radio astronomy.
Funding Information
  • Natural Sciences and Engineering Research Council of Canada (RGPIN-2019-06186)
  • Istituto Nazionale di Astrofisica (1.05.12.04.04)
  • United States - Israel Binational Science Foundation (2020203)
  • Israel Science Foundation (1679/23)
  • Department of Education and Training ∣ Australian Research Council (CE230100016)
  • Ministerio de Ciencia, Innovación y Universidades (PID2023-147883NB-C21)
  • MEC ∣ Agencia Estatal de Investigación (CEX2021-001131-S)
  • EC ∣ NextGenerationEU (RFF M4C21.1)
  • Ministero dell'Università e della Ricerca (202298J7KT - PEACE)

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