Do the Fermi Gamma-Ray Burst Monitor and Swift Burst Alert Telescope see the Same Short Gamma-Ray Bursts?

TL;DR

This study compares the short gamma-ray burst detections of Swift BAT and Fermi GBM, finding similar sensitivities and populations, and highlighting the potential for joint detection of gravitational wave counterparts.

Contribution

It provides a detailed comparison of SGRB detection capabilities and populations between Swift BAT and Fermi GBM, explaining differences and similarities in their observations.

Findings

Both instruments have similar average sensitivities.

No significant difference in the SGRB populations detected.

Ground searches can identify untriggered SGRBs for gravitational wave follow-up.

Abstract

Compact binary system mergers are expected to generate gravitational radiation detectable by ground-based interferometers. A subset of these, the merger of a neutron star with another neutron star or a black hole, are also the most popular model for the production of short gamma-ray bursts (GRBs). The Swift Burst Alert Telescope (BAT) and the Fermi Gamma-ray Burst Monitor (GBM) trigger on short GRBs (SGRBs) at rates that reflect their relative sky exposures, with the BAT detecting 10 per year compared to about 45 for GBM. We examine the SGRB populations detected by Swift BAT and Fermi GBM. We find that the Swift BAT triggers on weaker SGRBs than Fermi GBM, providing they occur close to the center of the BAT field-of-view, and that the Fermi GBM SGRB detection threshold remains flatter across its field-of-view. Overall, these effects combine to give the instruments the same average sensitivity, and account for the SGRBs that trigger one instrument but not the other. We do not find any evidence that the BAT and GBM are detecting significantly different populations of SGRBs. Both instruments can detect untriggered SGRBs using ground searches seeded with time and position. The detection of SGRBs below the on-board triggering sensitivities of Swift BAT and Fermi GBM increases the possibility of detecting and localizing the electromagnetic counterparts of gravitational wave events seen by the new generation of gravitational wave detectors.