A Radio Flare in the Long-Lived Afterglow of the Distant Short GRB 210726A: Energy Injection or a Reverse Shock from Shell Collisions?

TL;DR

This paper reports the discovery of a luminous, late-rising radio afterglow in a high-redshift short GRB, suggesting energy injection or reverse shocks as explanations, and highlights the importance of late-time observations.

Contribution

It presents the first detailed analysis of a late-time radio flare in a short GRB at high redshift, proposing models involving energy injection or reverse shocks to explain the phenomenon.

Findings

Radio afterglow brightened significantly around 11 days post-burst.

Standard forward shock models cannot explain the radio flare.

Energy injection or reverse shock models fit the broad-band data better.

Abstract

We present the discovery of the radio afterglow of the short $\gamma$-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of $z\sim 2.4$. While radio observations commenced $\lesssim 1~$day after the burst, no radio emission was detected until $\sim11~$days. The radio afterglow subsequently brightened by a factor of $\sim 3$ 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 $\approx 5$. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of $\approx 4$, or a reverse shock from a shell collision are viable solutions to match the broad-band behavior. At $z\sim 2.4$, 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.