TITLE: GCN CIRCULAR NUMBER: 39823 SUBJECT: GRB 250320B: Fermi GBM Observation DATE: 25/03/21 21:09:51 GMT FROM: eliza.neights@gmail.com

E. Neights (GWU, NASA/GSFC), R. Hamburg (UAH), C. Meegan (UAH) and O.J. Roberts (USRA) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 23:15:22.02 UT on 20 March 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250320B (trigger 764205327/250320969). which was also detected by AstroSat CZTI (Joshi et al. 2025, GCN 39808), SVOM/GRM (Zhang et al. 2025, GCN 39813), and Fermi LAT (Holzmann Airasca et al. 2025, GCN 39819). The Fermi GBM on-ground location is consistent with the LAT position.

The angle from the Fermi LAT boresight is 40 degrees.

The GBM light curve consists of multiple emission episodes with a duration (T90) of about 86 s (50-300 keV). The time-averaged spectrum from T0-2.0 to T0+107.5 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.16 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 570 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is (9.02 +/- 0.07)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+75 s in the 10-1000 keV band is 21.6 +/- 0.4 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak = 547 +/- 22 keV, alpha = -1.16 +/- 0.01 and beta = -2.7 +/- 0.2.

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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