Interpreting GRB170817A as a giant flare from a jet-less double neutron-star merger

TL;DR

This paper proposes that GRB170817A was caused by a giant flare from a jet-less neutron star merger, challenging the off-axis jet interpretation and suggesting a new quasi-isotropic emission mechanism.

Contribution

It introduces a giant-flare-like model for GRB170817A, emphasizing a relativistic outflow driven by magnetic fields without requiring a jet, and predicts similar emissions in other neutron star mergers.

Findings

The delay is incompatible with off-axis jet emission.

A relativistic outflow with a steep velocity profile explains the observations.

Most neutron star mergers may produce detectable hard X-ray emissions.

Abstract

We show that the delay between GRB170817A and GW170817 is incompatible with de-beamed emission from an off-axis relativistic jet. The prompt emission and the subsequent radio and X-ray observations can instead be interpreted within a giant-flare-like scenario, being the result of a relativistic outflow driven by the ultra-strong magnetic field produced by magnetohydrodynamic amplification during the merger of the progenitor double neutron-star binary. Within such picture, the data indicate that the outflow must be endowed with a steep velocity profile, with a relatively fast tail extending to Gamma~8. Since the conditions for the launch of such an outflow are quite general, and the presence of a velocity profile is a natural expectation of the acceleration process, most neutron star binary mergers should feature this quasi-isotropic, hard X-ray emission component, that can be a powerful guide to the discovery of additional kilonovae associated to relatively nearby gravitational wave events.