TITLE: GCN CIRCULAR NUMBER: 40346 SUBJECT: GRB 250504A: Fermi GBM Observation DATE: 25/05/05 08:00:47 GMT FROM: Elisabetta Bissaldi at Politecnico and INFN Bari elisabetta.bissaldi@ba.infn.it

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

"At 23:25:54.78 UT on 04 May 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250504A (trigger 768093959/250504976), which was also detected by Swift BAT (M. J. Moss et al. 2025, GCN 40343). The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 111 degrees.

The GBM light curve consists of multiple peaks with a duration (T90) of about 49 s (50-300 keV). The time-averaged spectrum from T0+0.003 to T0+59.393 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.06 +/- 0.08 and the cutoff energy, parameterized as Epeak, is 621 +/- 130 keV.

The event fluence (10-1000 keV) in this time interval is (1.20 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+10 s in the 10-1000 keV band is 3.6 +/- 0.3 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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