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GCN Circular 32642

Subject
GRB 221009A: Fermi GBM observation
Date
2022-10-10T04:04:41Z (a year ago)
From
Stephen Lesage at Fermi-GBM Team <sjl0014@uah.edu>
S. Lesage (UAH), P. Veres (UAH), O.J. Roberts (USRA),
E. Burns (LSU), and E. Bissaldi (Politecnico and INFN Bari)
report on behalf of the Fermi GBM Team:

"At 13:16:59.99 UT on 09 October 2022, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 221009A (trigger 687014224/221009553) which
was also detected by Swift-BAT (S. Dichiara, et al. 2022, GCN 32632;
J. A. Kennea, et al. 2022, GCN 32635), Fermi-LAT (E. Bissaldi et al. 2022,
GCN 32637), INTEGRAL (SPI-ACS), Konus-Wind, and triangulated by IPN
(D. Svinkin et al. 2022, GCN 32641). The GBM on-ground location (GCN 32636)
is
consistent with the Swift-BAT and Fermi-LAT locations and the IPN
localization.

The angle from the Fermi LAT boresight is 73 degrees.

The GBM light curve consists of two emission episodes, a single isolated
peak
followed by a longer, extremely bright, multi-pulsed emission episode with
a duration (T90) of about 327 s (10-1000 keV).

The time-averaged spectrum for the first emission episode
from T0-0.0 to T0+43.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.70 +/- 0.02 and the cutoff energy,
parameterized as Epeak, is 375 +/- 87 keV.
The event fluence (10-1000 keV) in this time interval is
(2.12 +/- 0.05)E-05 erg/cm^2.

Due to the brightness of the second emission episode, the likelihood of
pulse pile-up and other systematic effects are very high and no single
spectral model provides an adequate fit in this preliminary analysis.
The event fluence (10-1000 keV) from T0+175 to T0+1458 s
is on the order of (2.912 +/- 0.001)E-02 erg/cm^2.
The 1.024 sec peak photon flux measured
starting from T0+238.8 s in the 10-1000 keV band
is on the order of 2385 +/- 3 ph/s/cm^2,
making this the most intense and fluent GRB detected by Fermi GBM.

Further analysis is being performed.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support
Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
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