Analysis of Sub-threshold Short Gamma-ray Bursts in Fermi GBM Data

TL;DR

This paper presents a method to detect sub-threshold short gamma-ray bursts in Fermi GBM data, enhancing sensitivity to weak events and aiding multi-messenger astrophysics, especially in conjunction with LIGO/Virgo gravitational wave detections.

Contribution

The study introduces a coherent, detector-response-aware search method for sub-threshold SGRBs in GBM data, improving detection of weak and off-axis gamma-ray events.

Findings

Successfully recovers BAT-detected SGRBs in GBM data.

Enhances detection sensitivity to weak gamma-ray signals.

Identifies potential gamma-ray counterpart to GW150914.

Abstract

The Fermi Gamma-ray Burst Monitor (GBM) is currently the most prolific detector of Gamma-Ray Bursts (GRBs). Recently the detection rate of short GRBs (SGRBs) has been dramatically increased through the use of ground-based searches that analyze GBM continuous time tagged event (CTTE) data. Here we examine the efficiency of a method developed to search CTTE data for sub-threshold transient events in temporal coincidence with LIGO/Virgo compact binary coalescence triggers. This targeted search operates by coherently combining data from all 14 GBM detectors by taking into account the complex spatial and energy dependent response of each detector. We use the method to examine a sample of SGRBs that were independently detected by the Burst Alert Telescope on board the Neil Gehrels Swift Observatory, but which were too intrinsically weak or viewed with unfavorable instrument geometry to initiate an on-board trigger of GBM. We find that the search can successfully recover a majority of the BAT detected sample in the CTTE data. We show that the targeted search of CTTE data will be crucial in increasing the GBM sensitivity, and hence the gamma-ray horizon, to weak events such as GRB 170817A. We also examine the properties of the GBM signal possibly associated with the LIGO detection of GW150914 and show that it is consistent with the observed properties of other sub-threshold SGRBs in our sample. We find that the targeted search is capable of recovering true astrophysical signals as weak as the signal associated with GW150914 in the untriggered data.