A candidate coherent radio flash following a neutron star merger

TL;DR

This paper reports the detection of a candidate coherent radio flash following a neutron star merger, suggesting prolonged central engine activity and possibly indicating a magnetar formation.

Contribution

It presents the first rapid follow-up observation of a short GRB with LOFAR, identifying a candidate radio counterpart with dispersion properties consistent with a neutron star merger.

Findings

Detected a 5.6σ radio flash at 144 MHz 76.6 mins after the GRB

The radio flash is highly dispersed, implying a redshift of about 0.58

The event's luminosity suggests a highly magnetised, rapidly spinning neutron star

Abstract

In this paper, we present rapid follow-up observations of the short GRB 201006A, consistent with being a compact binary merger, using the LOw Frequency ARray (LOFAR). We have detected a candidate 5.6$\sigma$, short, coherent radio flash at 144 MHz at 76.6 mins post-GRB with a 3$\sigma$ duration of 38 seconds. This radio flash is 27 arcsec offset from the GRB location, which has a probability of occurring by chance of $\sim$0.05% (3.8$\sigma$) when accounting for measurement uncertainties. Despite the offset, we show that the probability of finding an unrelated transient within 40 arcsec of the GRB location is $<10^{-6}$ and conclude that this is a candidate radio counterpart to GRB 201006A. We performed image plane dedispersion and the radio flash is tentatively (2.4$\sigma$) shown to be highly dispersed, allowing a distance estimate, corresponding to a redshift of $0.58\pm0.06$. The corresponding luminosity of the event at this distance is $6.7^{+6.6}_{-4.4} \times 10^{32}$ erg s$^{-1}$ Hz$^{-1}$. If associated with GRB 201006A, this emission would indicate prolonged activity from the central engine that is consistent with being a newborn, supramassive, likely highly magnetised, millisecond spin neutron star (a magnetar).