External Inverse-Compton Emission Associated with Extended and Plateau Emission of Short Gamma-Ray Bursts: Application to GRB 160821B

TL;DR

This paper models the external inverse-Compton process as a source of very-high-energy gamma rays in short gamma-ray bursts, explaining recent observations and predicting detectable signals for future telescopes.

Contribution

It applies the EIC scenario to GRB 160821B, demonstrating its potential to produce observable VHE gamma-ray emission during extended and plateau phases.

Findings

EIC flux can reach ~10^{-12} erg cm^{-2} s^{-1} within 10^3-10^4 seconds.

Predicted VHE emission peaks around the end of seed emission.

Future detectors like CTA can detect these VHE signals with higher significance.

Abstract

The recent detection of TeV photons from two gamma-ray bursts (GRBs), GRB 190114C and GRB 180720B, has opened a new window for multi-messenger and multi-wavelength astrophysics of high-energy transients. We study the origin of very-high-energy (VHE) $\gamma$-rays from the short GRB 160821B, for which the MAGIC Collaboration reported a $\sim 3 \sigma$ statistical significance. Short GRBs are often accompanied by extended and plateau emission, which is attributed to internal dissipation resulting from activities of a long-lasting central engine, and Murase et al. (2018) recently suggested the external inverse-Compton (EIC) scenario for VHE counterparts of short GRBs and neutron star mergers. Applying this scenario to GRB 160821B, we show that the EIC flux can reach $\sim 10^{-12}\rm~erg~cm^{-2}~s^{-1}$ within a time period of $\sim 10^3 - 10^4\rm~s$, which is consistent with the MAGIC observations. EIC $\gamma$-rays expected during the extended and plateau emission will be detectable with greater significance by future detectors such as the Cherenkov Telescope Array (CTA). The resulting light curve has a distinguishable feature, where the VHE emission is predicted to reach the peak around the end of the seed emission.