GCN Circular 29110
Subject
Fermi GBM Sub-Threshold Detection of GRB 201221A
Event
Date
2020-12-21T22:28:27Z (5 years ago)
From
Peter Veres at UAH <veresp@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
"
[GCN OPS NOTE(22Dec2020): The 5th paragraph contains some errors
in the number results. Please see Circ 29111 for the correct values.]