2023/08/21 by Schroeder, Genevieve, Rhodes, Lauren, Laskar, Tanmoy +32 · 4 citations
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2308.10936
We present the discovery of the radio afterglow of the short γ-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of z∼ 2.4. While radio observations commenced \lesssim 1~day after the burst, no radio emission was detected until ∼11~days. The radio afterglow subsequently brightened by a factor of ∼ 3 in the span of a week, followed by a rapid decay (a "radio flare"). We find that a forward shock afterglow model cannot self-consistently describe the multi-wavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of ≈ 5. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of ≈ 4, or a reverse shock from a shell collision are viable solutions to match the broad-band behavior. At z∼ 2.4, GRB 210726A is among the highest redshift short GRBs discovered to date as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by \lesssim 10~days after the burst, potentially missing these late rising, luminous radio afterglows.