Search for radio remnants of nearby off-axis Gamma-Ray Bursts in a sample of Swift/BAT events

TL;DR

This study searches for radio remnants of nearby off-axis gamma-ray bursts in a sample of Swift/BAT events, aiming to identify potential late-time radio signatures indicative of off-axis GRBs.

Contribution

It presents the first systematic search for late-time radio signals from nearby off-axis short GRBs in a specific Swift/BAT sample, expanding methods to detect hidden GRB populations.

Findings

Detected a potential radio source near GRB 130626 consistent with a neutron star radio flare at ~100 Mpc.

Identified the challenge of false positives due to unrelated persistent radio sources.

Highlights the need for further follow-up to confirm the nature of the radio source.

Abstract

The multi-messenger discovery of gravitational waves (GWs) and light from the binary neutron star (NS) merger GW170817, associated with Gamma-Ray Burst (GRB) 170817A and kilonova AT2017gfo, has marked the start of a new era in astrophysics. GW170817 has confirmed that binary NS mergers are progenitors of at least some short GRBs. The peculiar properties of the GRB 170817A radio afterglow, characterized by a delayed onset related to the off-axis geometry, have also demonstrated how some nearby short GRBs may not be identified as such with standard short-timescale electromagnetic follow-up observations. Building upon this new information, we performed late-time radio observations of a sample of four short GRBs with unknown redshift and no previously detected afterglow in the \textit{Swift}/BAT sample in order to identify nearby ($d_L\lesssim 200$ Mpc) off-axis GRB candidates via their potential late-time radio signatures. We find a previously uncatalogued radio source within the error region of GRB 130626 with a $3-6$ GHz flux density consistent with a NS radio flare at a distance of $\sim 100$ Mpc. However, an origin related to a persistent radio source unrelated to the GRB cannot be excluded given the high chance of false positives in error regions as large as those considered here. Further radio follow-up observations are needed to better understand the origin of this source.