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GCN Circular 30779

Fermi GBM Sub-Threshold Detection of GRB 210905A
2021-09-06T05:39:59Z (2 years ago)
Peter Veres at UAH <>
P. Veres (UAH) and O. J. Roberts (USRA) report
on behalf of the Fermi-GBM Team:

"Swift-BAT detected GRB 210905A at 00:12:41 UT (GCN 30765). There was no
Fermi-GBM onboard trigger around the event.

The GBM targeted search [1], the most sensitive, coherent search for
GRB-like signals was run from +/-30 s around the BAT trigger time.  A
transient source was identified whose most significant timescale according
to the search is 8.192 s with a false alarm rate of 1.3e-5 Hz, and a
location consistent with the Swift-BAT event, using the standard search
protocol with a S/N of 13.  The GBM targeted search event was found with
the highest significance with a "normal" spectral template (Band function
with Epeak = 230 keV, alpha = -1.0, beta = -2.3).

The GBM light curve consists of multiple overlapping pulses. The
time-averaged spectrum from T0-26 s to T0+16 s is best fit by a power law
function with an exponential high-energy cutoff (T0 is 2021-09-05 00:12:41
UT). The power law index is -1.19 +/- 0.13 and the cutoff energy,
parameterized as Epeak, is 204 +/- 45 keV.

The event fluence (10-1000 keV) in this time interval for the best fitting
model is (6.08 +/- 0.59)E-6 erg/cm^2. Using the redshift z=6.318 reported
by Tanvir et al. (GCN 30771), we derive an isotropic equivalent energy in
the 1-10,000 keV range of (4.25 +/- 0.41)E+53 erg.

This analysis is preliminary.

[1] Goldstein et al. 2019 arXiv:1903.12597
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