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

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 groundbased 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 onboard 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.

Search for advanced LIGO single interferometer compact binary coalescence signals in coincidence with Gamma-ray events in Fermi-GBM

“…As such, in order to get Q G (LLR), one needs the distribution of noise and signals with respect to the LLR. A sample of real signals [23] was used to create a histogram of LLR. The distribution was fit using a kernel density estimation (KDE) from LLR = 5 (sufficiently small threshold in order to be sure of not missing any interesting event) to LLR = 2000 (this threshold is imposed by the quality of the KDE fitting).…”

Fermi-GBM Follow-up of LIGO-Virgo Binary Black Hole Mergers: Detection Prospects

“…This event did not trigger GBM, but was found in an off-line search (e.g. Blackburn et al 2015;Goldstein et al 2016;Kocevski et al 2018;Goldstein et al 2019). Having been identified using a hard spectral template and lasting ∼ 1 s, GW150914-GBM is consistent with a short GRB.…”

“…GW150914-GBM was found in an off-line search. Off-line searches improve the sensitivity by a factor of few (Kocevski et al 2018), thus improving (decreasing) the BBH-to-GRB ratio.…”

Fermi: Monitoring the Gamma-Ray Universe

“…This capability remains in case of future near-miss situations. The operation of the GBM originally transmitted time-tagged event data only when an on-board trigger enabled this mode, but a change in 2012 switched to continuous time-tagged event data, allowing more sensitive searches for short γ-ray bursts [4]. Based on flight experience, the LAT team developed a new analysis process, Pass 8, that increased the rate of detected γ rays by about 30 percent and with improved angular resolution [5].…”