TITLE: GCN CIRCULAR NUMBER: 39795 SUBJECT: GRB 250321A: Fermi GBM Observation DATE: 25/03/21 06:04:23 GMT FROM: A. Holzmann Airasca at University of Trento and INFN Bari a.holzmannairasca@unitn.it

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

"At 00:42:39.02 UT on 21 March 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250321A (trigger 764210564/250321030), which was also detected by Swift BAT (Gupta et al. 2025, GCN 39794). The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 39 degrees.

The GBM light curve shows one main pulse with a duration (T90) of about 6 s (50-300 keV). The time-averaged spectrum from T0-0.5 to T0+7.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.83 +/- 0.20 and the cutoff energy, parameterized as Epeak, is 101 +/- 12 keV.

The event fluence (10-1000 keV) in this time interval is (1.04 +/- 0.08)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0-0.13 s in the 10-1000 keV band is 4.7 +/- 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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