TITLE: GCN CIRCULAR NUMBER: 42410 SUBJECT: GRB 251022A: Fermi GBM Observation DATE: 25/10/23 22:45:23 GMT FROM: oindabimukherjee@gmail.com

O. Mukherjee (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 22:34:15.26 UT on 22 October 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251022A (trigger 782865260/251022940) which was also detected by Fermi-LAT ( N. Di Lalla et al. 2025, GCN 42384), MASTER OT (V. Lipunov et al. 2025, GCN 42395), and Swift/BAT-GUANO (James DeLaunay et al. 2025, GCN 42409). The Fermi GBM on-ground location is consistent with the Fermi-LAT position.

The angle from the Fermi LAT boresight is 36.5 degrees.

The GBM light curve consists of multiple emission episodes with a duration (T90) of about 99.6 s (50-300 keV). The time-averaged spectrum from T0-1 to T0+107.5 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.88 +/- 0.04 and the cutoff energy, parameterized as Epeak, is 154 +/- 5 keV.

The event fluence (10-1000 keV) in this time interval is (2.16 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+100 s in the 10-1000 keV band is 12.4 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak= 138 +/- 7 keV, alpha = -0.8 +/- 0.05 and beta = -2.6 +/- 0.2.

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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