Upscattered Cocoon Emission in Short Gamma-ray Bursts as High-energy Gamma-ray Counterparts to Gravitational Waves

TL;DR

This paper explores how prolonged jet activity in short gamma-ray bursts can produce high-energy gamma-ray counterparts to gravitational waves through cocoon interactions, with detectable GeV-TeV emissions.

Contribution

It introduces a model for high-energy gamma-ray production from cocoon interactions in short GRBs, linking prolonged jet activity to observable gamma-ray counterparts to GWs.

Findings

GeV-TeV gamma-rays can be produced with durations of 10^2-10^5 seconds.

High-energy gamma-ray spectra are calculated using a one-zone steady-state approximation.

Detection prospects include Fermi/LAT and CTA observations.

Abstract

We investigate prolonged engine activities of short gamma-ray bursts (SGRBs), such as extended and/or plateau emissions, as high-energy gamma-ray counterparts to gravitational waves (GWs). Binary neutron-star mergers lead to relativistic jets and merger ejecta with $r$-process nucleosynthesis, which are observed as SGRBs and kilonovae/macronovae, respectively. Long-term relativistic jets may be launched by the merger remnant as hinted in X-ray light curves of some SGRBs. The prolonged jets may dissipate their kinetic energy within the radius of the cocoon formed by the jet-ejecta interaction. Then the cocoon supplies seed photons to non-thermal electrons accelerated at the dissipation region, causing high-energy gamma-ray production through the inverse Compton scattering process. We numerically calculate high-energy gamma-ray spectra in such a system using a one-zone and steady-state approximation, and show that GeV--TeV gamma-rays are produced with a duration of $10^2-10^5$ seconds. They can be detected by {\it Fermi}/LAT or CTA as gamma-ray counterparts to GWs.