TITLE: GCN CIRCULAR NUMBER: 40004 SUBJECT: GRB 250331D: Fermi GBM observation DATE: 25/04/01 13:33:45 GMT FROM: Elisabetta Bissaldi at Politecnico and INFN Bari elisabetta.bissaldi@ba.infn.it

E. Bissaldi (Politecnico and INFN Bari), C. Meegan (UAH) and A. Holzmann Airasca (Univ. Trento and INFN Bari) report on behalf of the Fermi GBM Team:

"At 05:49:17.01 UT on 31 March 2025, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 250331D (trigger 765092962 / 250331243), which was also detected by the Swift/BAT-GUANO (Ronchini et al. 2025, GCN 39979). The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 27 degrees.

The GBM light curve consists of a single emission episode with a duration (T90) of about 9 s (50-300 keV). The time-averaged spectrum from T0-2.5 s to T0+6.4 s is adequately fit by a power law function with an exponential high-energy cutoff. The power law index is -1.12 +/- 0.27 and the cutoff energy, parameterized as Epeak, is 91 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is (6.1 +/- 0.8)E-07 erg/cm^2. The 1-sec peak photon flux measured starting from T0-0.64 s in the 10-1000 keV band is 2.2 +/- 0.2 ph/s/cm^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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