TITLE:   GCN CIRCULAR
NUMBER:  39033
SUBJECT: GRB 250118A: Fermi GBM Observation
DATE:    25/01/25 20:29:13 GMT
FROM:    Peter Veres at University of Alabama in Huntsville <veresp@gmail.com>

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

"At 18:31:11.97 UT on 18 January 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250118A (trigger 758917876/250118772).
which was also detected by Swift-BAT/GUANO (Ronchini et al. 2025, GCN 39004).
The Fermi GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight is 14 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 0.3 s (50-300 keV). The time-averaged spectrum
from T0-0.13 to T0+0.06 s is best fit by
a simple power law function with index -1.18 +/- 0.06.

The event fluence (10-1000 keV) in this time interval is
(4.1 +/- 0.4)E-07 erg/cm^2. The 64-ms peak photon flux measured
starting from T0-0.064 s in the 10-1000 keV band is 7.4 +/- 0.9 ph/s/cm^2.

A power law function with an exponential high-energy cutoff gives an acceptable fit to the spectrum.
The power law index is -0.73 +/- 0.22 and the cutoff energy,
parameterized as Epeak, is 1100 +/- 500 keV.

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/"