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GCN Circular 31291

Fermi GBM Sub-Threshold Detection of GRB 211221A
2021-12-23T15:05:00Z (2 years ago)
Cori Fletcher at USRA <>
C. Fletcher (USRA) reports on behalf of the Fermi-GBM Team:

Swift-BAT detected GRB 211221A at 20:48:40 UT (GCN 31273). There was no
Fermi-GBM onboard trigger around the event.

An automated, blind search for short gamma-ray bursts below the onboard
triggering threshold in Fermi-GBM identified no counterparts.

The GBM targeted search [1], the most sensitive, coherent search for
GRB-like signals identified a transient most significantly on the 8.192 s
timescale, with a location consistent with the Swift-BAT event and
a S/N of 21, using the standard search protocol.
The GBM targeted search event was found with the highest
significance with a "normal" spectrum (Band function with
Epeak = 230 keV, alpha = -1.0, beta = -2.3) for a GRB.

[1] Goldstein et al. 2019 arXiv:1903.12597
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