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/"

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