Search for Radio Remnants of Nearby Off-axis Gamma-Ray Bursts in a Sample of Swift/BAT Events

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 Swift/BAT sample in order to identify nearby ( 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 flux density consistent with an NS radio flare at a distance of ∼100 Mpc. An origin related to a persistent radio source unrelated to the GRB cannot be excluded nor confirmed given the high chance of false positives in error regions as large as those considered here, and the limited time baseline of our observations. Further radio (and X-ray) follow-up observations are needed to better understand the origin of this source.

Radio Constraints on r-process Nucleosynthesis by Collapsars

“…Collapsar wind ejecta could produce late-time, slowly evolving radio flares driven by the outflow's interaction with the surrounding medium. This radio emission is analogous to the late-time, slowly rising radio flares expected in the case of neutron star mergers (Nakar & Piran 2011;Bartos et al 2019;Grandorf et al 2021), which may have already been detected (Lee et al 2020;Hajela et al 2022). Collapsars, however, may be more favorable for the detection of radio flares: (i) since they are rarer than neutron star mergers, they individually need to eject more matter than a neutron star merger to explain the observed abundance of r-process elements in the universe, and (ii) they are typically found in a denser interstellar medium (ISM) than neutron star mergers, potentially leading to brighter radio flares.…”

A Search for Kilonova Radio Flares in a Sample of Swift/BAT Short Gamma-Ray Bursts

“…However, these results have spurred new interest in the search for so-called late-time radio flares from short GRBs (e.g., Nakar & Piran 2011;Hotokezaka & Piran 2015;Hotokezaka et al 2018;Kathirgamaraju et al 2019;Nedora et al 2021), especially considering that the kilonova afterglow could be visible in radio for years after the initial event. Several recent efforts have targeted both well-localized short GRBs with known redshifts and short GRBs lacking accurate X-ray localizations and redshift measurements (e.g., Schroeder et al 2020;Bruni et al 2021;Grandorf et al 2021;Ricci et al 2021;Ghosh et al 2022).…”

GW170817 4.5 Yr After Merger: Dynamical Ejecta Afterglow Constraints

“…With GW170817, these late-time rebrightening models have spurred new interest in the community (Hotokezaka et al 2018;Bartos et al 2019;Kathirgamaraju et al 2019;Liu et al 2020;Margalit & Piran 2020), especially given their potential to probe the nature of the merger remnant in relation to the equation of state (EoS) of nuclear matter (see, e.g., Nedora et al 2021, and references therein). Thus, additional observational campaigns have been carried out in search for late-time radio afterglows for both GW170817 (Balasubramanian et al 2021;Hajela et al 2022;Troja et al 2022) and other short GRBs (e.g., Klose et al 2019;Bruni et al 2021;Grandorf et al 2021;Ricci et al 2021), albeit without any definitive detections so far.…”