TITLE: GCN CIRCULAR NUMBER: 39682 SUBJECT: GRB 250223B: Fermi GBM Observation DATE: 25/03/12 10:06:11 GMT FROM: Rushikesh Sonawane at IISER, TVM rushikesh23@iisertvm.ac.in

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

"At 10:19:23.76 UT on 23 February 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250223B (trigger 761998768/250223430). which was also detected by Swift/BAT-GUANO (DeLaunay et al. 2025, GCN 39671). The Fermi GBM on-ground location is consistent with the Swift/BAT position.

The angle from the Fermi LAT boresight is 56 degrees.

The GBM light curve consist of multiple spikes with a duration (T90) of about 2.4 s (50-300 keV). The time-averaged spectrum from T0-1.0 to T0+1.7 s is best fit by a simple power law function with index -1.24 +/- 0.07.

The event fluence (10-1000 keV) in this time interval is (3.9 +/- 0.6)E-07 erg/cm^2. The 64-ms peak photon flux measured starting from T0+1.2 s in the 10-1000 keV band is 2.9 +/- 1.1 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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