[vsnet-grb-info 27007] Fermi GBM Sub-Threshold Detection of GRB 201221A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29110
SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 201221A    
DATE:    20/12/21 22:28:27 GMT
FROM:    Peter Veres at UAH  <veresp at gmail.com>

P. Veres (UAH) reports on behalf of the Fermi-GBM Team:

"At 07:09:01 UT on 21 December, 2020 Swift detected GRB 201221A  (Page
et al. GCN 29096). 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 a long
counterpart with high significance.  This was not included in the
public list (https://gcn.gsfc.nasa.gov/gcn/fermi_gbm_subthresh_archive.html),
because the search reports only short candidates.

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 log likelihood ratio of 52 and a location
consistent with the event.

The GBM light curve consists of a single peak. The time-averaged
spectrum from T0-10 s to T0+47 s is best fit by a power law function
with an exponential high-energy cutoff (T0 is the Swift trigger time).
The power law index is -1.17 +/- 0.21 and the cutoff energy,
parameterized as Epeak, is 129 +/- 28 keV.

The event fluence (10-1000 keV) in this time interval is (3.30 +/-
0.27)E-04 erg/cm^2. Using the redshift z=5.7 reported by Malesani et
al (GCN 29100),  we derive an isotropic equivalent energy in the
1-10,000 keV range of (2.02 +/- 0.16)E+55 erg.

This analysis is preliminary.

[1] Goldstein et al. 2019 arXiv:1903.12597
"