Off-axis short GRBs from structured jets as counterparts to GW events

TL;DR

This paper proposes that structured jets in binary neutron star mergers can produce detectable off-axis short GRBs as electromagnetic counterparts to gravitational wave events, expanding the range of observable signals beyond on-axis emissions.

Contribution

It introduces a realistic jet model with smooth angular variation, demonstrating the potential to detect off-axis short GRBs as GW counterparts, which was previously considered unlikely.

Findings

Off-axis short GRBs can be detected with current instruments.

Structured jets increase the observable angle range for EM counterparts.

Simulations support detection feasibility with LIGO triggers.

Abstract

Binary neutron star mergers are considered to be the most favorable sources that produce electromagnetic (EM) signals associated with gravitational waves (GWs). These mergers are the likely progenitors of short duration gamma-ray bursts (GRBs). The brief gamma-ray emission (the "prompt GRB" emission) is produced by ultra-relativistic jets, as a result, this emission is strongly beamed over a small solid angle along the jet. It is estimated to be a decade or more before a short GRB jet within the LIGO volume points along our line of sight. For this reason, the study of the prompt signal as an EM counterpart to GW events has been sparse. We argue that for a realistic jet model, one whose luminosity and Lorentz factor vary smoothly with angle, the prompt signal can be detected for a significantly broader range of viewing angles. This can lead to a new type of EM counterpart, an "off-axis" short GRB. Our estimates and simulations show that it is feasible to detect these signals with the aid of the temporal coincidence from a LIGO trigger, even if the observer is substantially misaligned with respect to the jet.