GCN Circular 39063

A. Myers (NPP/GSFC) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 16:22:53.77 UT on 28 January 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250128B (trigger 759774178/250128683). which was also detected by Swift BAT (P.A Evans et al. 2025, GCN 39058). The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 86 degrees.

The GBM light curve consists of a short single emission episode with a duration (T90) of about 1.1 s (50-300 keV). The time-averaged spectrum from T0-0.8 to T0+0.4 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.0 +/- 0.5 and the cutoff energy, parameterized as Epeak, is 176 +/- 115 keV. A power law function with index -1.6 +/- 0.2 fits the spectrum equally well.

The event fluence (10-1000 keV) in this time interval is (2.1 +/- 0.7)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 9 +/- 2 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/"