TITLE: GCN CIRCULAR NUMBER: 41301 SUBJECT: GRB 250808A: Fermi GBM Observation DATE: 25/08/09 12:34:41 GMT FROM: A. Holzmann Airasca at University of Trento and INFN Bari a.holzmannairasca@unitn.it

A. Holzmann Airasca (UniTrento and INFN Bari), S. Bala (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 18:25:12.48 UT on 08 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250808A (trigger 776370317/250808768). which was also detected by SVOM/ECLAIRs (D. Adrien et al. 2025, GCN 41297). The Fermi GBM on-ground location is consistent with the SVOM/ECLAIRs position.

The angle from the Fermi LAT boresight is 33 degrees.

The GBM light curve consists of a single variable emission episode with a duration (T90) of about 49 s (50-300 keV). The time-averaged spectrum from T0-6.1 to T0+51.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.1 +/- 0.2 and the cutoff energy, parameterized as Epeak, is 70 +/- 7 keV.

The event fluence (10-1000 keV) in this time interval is (2.7 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+45 s in the 10-1000 keV band is 2.5 +/- 0.2 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak= 55 +/- 9 keV, alpha = -0.7 +/- 0.3 and beta = -2.5 +/- 0.3.

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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