TITLE: GCN CIRCULAR NUMBER: 39680 SUBJECT: GRB 250228A: Fermi GBM Observation DATE: 25/03/12 07:13:25 GMT FROM: Utkarsh Pathak at IIT Bombay utkarshpathak.07@gmail.com

U. Pathak (IIT Bombay), M. Dafčíková (Masaryk U.), and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 07:23:46.16 UT on 28 February 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250228A (trigger 762420231/250228308). which was also detected by Swift/BAT-GUANO (Ronchini et al. 2025, GCN 39674). The Fermi GBM on-ground location is consistent with the Swift/BAT position.

The angle from the Fermi LAT boresight is 27 degrees.

The GBM light curve consists of a weak pulse from a single emission episode with a duration (T90) of about 0.6 s (50-300 keV). The time-averaged spectrum from T0-0.5 to T0+0.3 s is best fit by a simple power law function with index -1.45 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is (2.0 +/- 0.3)E-07 erg/cm^2. The 64-ms peak photon flux measured starting from T0-0.064 s in the 10-1000 keV band is 4.3 +/- 0.9 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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