GCN Circular 42530

U. Pathak (IITB), R. Sonawane (IISER TVM) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 07:21:00.49 UT on 31 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251031A (trigger 783588065/251031306).
which was also detected by Swift/BAT-GUANO (DeLaunay et al. 2025, GCN 42528
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 ).
The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.

The angle from the Fermi LAT boresight is 49 degrees.

The GBM light curve consists of multiple spikes from a single emission episode 
with a duration (T90) of about 45 s (50-300 keV). The time-averaged
spectrum from T0-1.5 to T0+47.1 s is best fit by a power law
function with an exponential high-energy cutoff.
The power law index is -0.7 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 130 +/- 1 keV.

The event fluence (10-1000 keV) in this time interval is
(8 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+23 s in the 10-1000 keV band is 8 +/- 1 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 116 +/- 1 keV, alpha = -0.59 +/- 0.01 and beta = -2.51 +/- 0.04.

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/"