Untriggered Swift-GRBs in Fermi/GBM data

TL;DR

This paper develops methods to identify untriggered gamma-ray bursts in Fermi/GBM data, revealing a significant population of faint GRBs missed by standard triggers, and compares their properties to triggered GRBs.

Contribution

It introduces new algorithms for detecting untriggered GRBs in Fermi/GBM data and analyzes their spectral properties and occurrence rate.

Findings

Untriggered GRBs constitute about 7% of all GBM-detected GRBs.

Estimated rate of untriggered GRBs is approximately 1.6 per month.

Spectral properties of untriggered GRBs are compared to triggered ones.

Abstract

The Fermi Gamma-Ray Burst Monitor (GBM) onboard the Fermi spacecraft currently operates on several trigger algorithms on various time scales and energy ranges. Motivated by the pursuit of faint Gamma-Ray Bursts (e.g. the elusive class of postulated low-luminosity GRBs), here we present the search for untriggered GRBs in the GBM data stream. To this end, I will demonstrate the methods and algorithms which have been developed by the GBM team. As a preliminary result, I am going to highlight the spectral analysis of GRBs which triggered the Swift satellite, but not GBM, and came from positions above the horizon, with a favorable orientation to at least one GBM detector. The properties of these GRBs are then compared to the full sample of GBM GRBs published in the GBM spectral catalogue. We estimate that the lower limit for untriggered GRBs in the GBM data is about 1.6 GRBs per month which corresponds to about 7% of the triggered GRBs