GCN Circular 31791
Subject
Fermi GBM Sub-Threshold Detection of GRB 220325A
Date
2022-03-26T03:41:37Z (2 years ago)
From
Joshua Wood at MSFC/Fermi-GBM <joshua.r.wood@nasa.gov>
J. Wood (NASA/MSFC) reports on behalf of the Fermi-GBM Team:
There was no Fermi-GBM onboard trigger around the time of GRB 220325A,
which was detected by Swift-BAT at 17:16:23 UT on 25 March, 2022 (Ferro
et al. GCN 31787).
An automated, blind search for short gamma-ray bursts below the onboard
triggering threshold in Fermi-GBM identified GRB 220325A with medium
reliability [1], with a localization consistent with the Swift-BAT
location at the ~2 sigma level, and a discovery timescale of 2.751 s.
The start time of the GBM sub-threshold detection coincides with the time
of the peak photon flux observed by Swift-BAT.
The GBM targeted search [2], the most sensitive, coherent search for
GRB-like signals also identified a transient most significantly on the 2.048 s
timescale, with a location consistent with the Swift-BAT event at
the ~2 sigma level and a signal-to-noise ratio of 12.3,
using the standard search protocol.
The GBM targeted search event was found with the highest
significance with a "soft" spectrum (Band function with
Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.
This analysis is preliminary.
[1] https://gcn.gsfc.nasa.gov/notices_gbm_sub/669921389.fermi
[2] Goldstein et al. 2019 arXiv:1903.12597