LOFAR early-time search for coherent radio emission from GRB 180706A

TL;DR

This study used LOFAR to search for coherent radio emission from GRB 180706A shortly after the burst, setting the deepest limits yet and finding no evidence of such emission, thus constraining models of GRB central engines.

Contribution

First deep LOFAR rapid response observations of a GRB to search for coherent radio emission, providing new upper limits and constraining theoretical models.

Findings

No coherent radio emission detected to 1.7 mJy limit.

Marginal 1 sigma peak flux density at GRB position.

No short-duration FRB-like emission observed.

Abstract

The nature of the central engines of gamma-ray bursts (GRBs) and the composition of their relativistic jets are still under debate. If the jets are Poynting flux dominated rather than baryon dominated, a coherent radio flare from magnetic re-connection events might be expected with the prompt gamma-ray emission. There are two competing models for the central engines of GRBs; a black hole or a newly formed milli-second magnetar. If the central engine is a magnetar it is predicted to produce coherent radio emission as persistent or flaring activity. In this paper, we present the deepest limits to date for this emission following LOFAR rapid response observations of GRB 180706A. No emission is detected to a 3$\sigma$ limit of 1.7 mJy beam$^{-1}$ at 144 MHz in a two-hour LOFAR observation starting 4.5 minutes after the gamma-ray trigger. A forced source extraction at the position of GRB 180706A provides a marginally positive (1 sigma) peak flux density of $1.1 \pm 0.9$ mJy. The data were time-sliced into different sets of snapshot durations to search for FRB like emission. No short duration emission was detected at the location of the GRB. We compare these results to theoretical models and discuss the implications of a non-detection.