TITLE: GCN CIRCULAR NUMBER: 43046 SUBJECT: GRB 251208B: Fermi GBM Observation DATE: 25/12/09 11:51:58 GMT FROM: Andreas von Kienlin at MPE <azk@mpe.mpg.de> A. von Kienlin (MPE) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 10:17:17.29 UT on 08 December 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251208B (trigger 786881842/251208429). which was also detected by Fermi-LAT (E. Bissaldi et al. 2025, GCN 43034) and Swift/BAT-GUANO (GCN #43033). The Fermi GBM on-ground location was reported by the Fermi-GBM team in GCN 43028 and is consistent with the Fermi-LAT and Swift/BAT positions. The GBM light curve shows a structured emission episode with a duration (T90) of about 62.5 s (50-300 keV). The time-averaged spectrum from T0-4.1 to T0+61.4 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.67 +/- 0.08 and the cutoff energy, parameterized as Epeak, is 250 +/- 20 keV. The event fluence (10-1000 keV) in this time interval is (9.5 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+55 s in the 10-1000 keV band is 2 +/- 0.2 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/43046. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...