TITLE: GCN CIRCULAR NUMBER: 39063 SUBJECT: GRB 250128B: Fermi GBM Observation DATE: 25/01/28 22:42:25 GMT FROM: Ava Myers at NASA GSFC <ava.myers@nasa.gov> 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](https://gcn.nasa.gov/circulars/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/" View this GCN Circular online at https://gcn.nasa.gov/circulars/39063. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...