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GCN Circular 30540

Subject
Fermi GBM Sub-Threshold Detection of GRB 210726A
Date
2021-07-28T15:21:41Z (2 years ago)
From
Peter Veres at UAH <veresp@gmail.com>
P. Veres (UAH) reports on behalf of the Fermi-GBM Team:

"Swift-BAT detected GRB 210726A at 19:19:03 UT (GCN 30523). 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 512 ms, and a location consistent with the Swift-BAT
event.  The GBM targeted search event was found with the highest
significance with a "soft" spectral template (Band function with Epeak = 70
keV, alpha = -1.9, beta = -3.7).

The GBM light curve consists of a single peak. The time-averaged spectrum
from T0+0.6 s to T0+1.4 s is best fit by a power law function with index
-2.03 +/- 0.3 (T0 is 2021-07-26 19:19:03 UT). A power law function with an
exponential high-energy cutoff fits equally well, but it is not
statistically preferred over the power law;  the fit measures the power law
index 0.23 +/- 1.67 and the cutoff energy, parameterized as Epeak, is 41
+/- 10 keV.

The event fluence (10-1000 keV) in this time interval for the power law
with the exponential cutoff is (6.4 +/- 1.7)E-08 erg/cm^2. Using the
photometric redshift z=0.35 reported by Watson et al. (GCN 30534), we
derive an isotropic equivalent energy in the 1-10,000 keV range of (2.1 +/-
0.5)E+49 erg.

This analysis is preliminary.

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