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GCN Circular 33414

Subject
GRB 230307A: possibly the second highest GRB energy fluence ever identified
Date
2023-03-08T05:41:31Z (a year ago)
From
Eric Burns at LSU <erickayserburns@gmail.com>
E. Burns (LSU), A. Goldstein (USRA), S. Lesage (UAH), and S. Dalessi (UAH)
report on behalf of the Fermi-GBM Team.

The very bright long GRB 230307A has been detected by many prompt GRB
monitors (GCN Circ. 33405; Dalessi, GCN Circ. 33407; Dalessi and Roberts,
GCN Circ. 33411; GECAM detection: Xiong etal., GCN Circ. 33406; Solar
Orbiter STIX detection: Xiao and Krucker, GCN Circ. 33410; AGILE-MCAL
detection: Casentini et al., GCN Circ. 33412) at 15:44:07.

Fermi-GBM reports a preliminary 10-1000 keV fluence of
(2.951 +/- 0.004)E-03 erg/cm^2 (Dalessi and Roberts, GCNCirc. 33411) with
a best-fit Comptonized spectrum with Epeak = 936 +/- 3 keV and alpha
-1.07 +/- 0.01. Utilizing this spectrum, this corresponds to a bolometric
fluence of 4.56E-3 erg/cm^2, with negligible error.

For reported GRB energy fluence values, only GRB 221009A is higher.
GRB 130427A has a reported fluence of 2.83E-3 erg/cm^2, 840304 of
~2.8E-3 erg/cm^2, and 830801 of >2.0e-3erg/cm^2. Noting that these three
values have some uncertainty due to detector issues and that this initial
value for GRB 230307A is preliminary, from these base values GRB 230307A
has the second highest reported value.

Follow-up is thus strongly encouraged. The best current localization is
from the IPN, reported in GCN 33413.
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