Similar Fermi-GBM sGRBs to GW/sGRB 170817A in MeV-GeV energies

TL;DR

This paper identifies gamma-ray bursts similar to GW/sGRB 170817A in Fermi-GBM data using spectral features and clustering, estimating their rate and implications for gravitational wave counterparts.

Contribution

The study introduces a method combining hardness ratios and clustering to find sGRBs akin to GW/sGRB 170817A in Fermi-GBM data, estimating their occurrence rate.

Findings

Identified eight sGRBs similar to GW/sGRB 170817A in Fermi-GBM data.

Estimated up to five GW+sGRB events could be observed by end of LIGO O4 run.

Deviation from estimated rate may impact understanding of event evolution over distance.

Abstract

The rate of observed gravitational waves (GWs) from neutron star-neutron star (NS-NS) mergers detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) indicates the existence of more than one short gamma-ray bursts (sGRBs) similar to GW/sGRB 170817A within the total gamma-ray bursts (GRBs) recorded by satellite detectors such as BATSE, Fermi-Gamma-ray Burst Monitor (Fermi-GBM), and Swift-Burst Alert Telescope (Swift-BAT). We investigated sGRBs in the Fermi-GBM dataset based on their MeV-GeV $\gamma$-ray emission features, to identify sGRBs similar to sGRB 170817A. Any addition of such events can impact the rate of NS-NS CBC events observed by LIGO. SGRB 170817A exhibits two distinct emission components: a non-thermal peak and a thermal component. We adopted a multifaceted approach to identify analogous sGRBs, which involved computing the hardness ratios $HR_{1}$ and $HR_{2}$ and then clustering them via the K-means algorithm. Our further studies reveal the presence of eight such events in Fermi-GBM data, which will enable us to calculate the rate of electromagnetic (EM) counterparts associated with LIGO GW events (GW+sGRB events) across all observing runs. Giving an estimation, by the end of the $O_4$ LIGO run, there could be nearly 5 GW+sGRB events. Deviation from this number may raise concerns about our understanding of the evolution of such events over distance.