TITLE: GCN CIRCULAR NUMBER: 42582 SUBJECT: GRB 251103B: Fermi GBM observation DATE: 25/11/04 21:18:14 GMT FROM: atrigg2@lsu.edu A. C. Trigg (NPP, NASA MSFC) report on behalf of the Fermi GBM Team: "At 16:46:36.44 UT on 03 November 2025, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 251103B (trigger 783881201/251103699), which was also detected by the Swift/XRT (Evans et al. 2025, GCN 42557) The Fermi GBM Final Real-time Localization (GCN 42555) is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time is 70 degrees. The GBM light curve shows/consists of rapid multi-peaked emission with a duration (T90) of about 15 s (50-300 keV). The time-averaged spectrum from T0-0.00 s to T0+10.0 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.07 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 149 +/- 3 keV. The event fluence (10-1000 keV) in this time interval is (2.28 +/- 0.08)E-6 erg/cm^2. The 256-msec peak photon flux measured starting from T0+2.24 s in the 10-1000 keV band is 5.6 +/- 0.5 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 146 +/- 3 keV, alpha = -1.07 +/- 0.01, and beta = -3.3 +/- 0.06. 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/42582. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...