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GCN Circular 35044

GRB 231115A: Fermi Observations of a probable Magnetar Giant Flare from M82
2023-11-15T20:50:45Z (7 months ago)
Sarah Dalessi at UAH <>
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S. Dalessi (UAH), O.J. Roberts (USRA/NASA-MSFC), P. Veres (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 15:36:21.20 UT on 15 November 2023, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 231115A (trigger 721755386/231115650).
which was also detected by Integral (P. D'Avanzo et al. 2023, GCN 35036 and S.Mereghetti et al. 2023, GCN 35037).
The Fermi GBM Final Real-time Localization is reported in GCN 35035 and is consistent with the Integral position.

The angle from the Fermi LAT boresight is 38 degrees.

The GBM light curve single peak with a duration (T90)
of about 0.03 s (50-300 keV). The time-averaged spectrum
from T0-0.02 to T0+0.02 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is 0.5 +/- 0.2 and the cutoff energy,
parameterized as Epeak, is 580 +/- 60 keV.

The event fluence (10-1000 keV) in this time interval is
(6.3 +/- 0.4)E-07 erg/cm^2. The 64-ms peak photon flux measured
starting from T0+0.0 s in the 10-1000 keV band is 11 +/- 1 ph/s/cm^2.

Using the distance to M82 of 3.5 Mpc (P. D'Avanzo et al. 2023, GCN 35036), we derive an Eiso of 1.4e45 ergs and Liso of 1.9e46 erg s-1 (64 ms, 1-10,000 keV). This with the rise time of ~2 ms, hard spectral slope and soft-hard-soft spectral evolution of the event, are consistent with an MGF origin.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:"
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