Emission of cocoon afterglow for short Gamma Ray Burst : a counterpart of gravitational waves?

TL;DR

This paper models the hydrodynamic evolution and emission of cocoon afterglows from short Gamma Ray Bursts, proposing them as promising electromagnetic counterparts to gravitational wave events from binary neutron star mergers.

Contribution

It introduces a comprehensive hydrodynamic model for cocoon afterglows, including the mildly relativistic regime, and evaluates their potential as electromagnetic counterparts to GW events.

Findings

Cocoon afterglows can produce detectable signals in X-ray, optical, and radio wavelengths.

Cocoon afterglow emission is comparable to GRB afterglow emission, making it a promising counterpart.

The model enhances understanding of EM signals associated with GW events from BNS mergers.

Abstract

The three gravitational wave events detected by LIGO are opening a new era for high- energy astrophysics. Nevertheless, location of such events remain unknown. A promising solution to the localization problem is to find an electromagnetic (EM) counterpart of GW- generating events, such as binary neutron star mergers (BNS). Indeed, their GW emission will be above sensitivity threshold in the near future. BNS are also considered as short Gamma Ray Bursts (sGRB) progenitors. However, sGRB are highly beamed. In this study, we will therefore focus on another EM counterpart candidate: the cocoon afterglow. The propagation of the GRB jet inside the matter ejected by the BNS produces a cocoon. Then, similarly to the GRB afterglow, a cocoon afterglow is produced, but with a mildly relativistic velocity. Firstly, we propose a model that gives the full hydrodynamic evolution of the cocoon including the mildly relativistic regime. Then we calculate the cocoon afterglow emission in X-ray, optical and radio wavelengths. Finally, we compare the cocoon afterglow emission to the GRB afterglow emission and conclude that the cocoon afterglow is a promising EM counterpart.