TITLE: GCN CIRCULAR NUMBER: 43174 SUBJECT: GRB 251221A: Fermi GBM Observation DATE: 25/12/21 20:13:03 GMT FROM: A. Holzmann Airasca at University of Trento and INFN Bari <a.holzmannairasca@unitn.it> A. Holzmann Airasca (UniTrento and INFN Bari) and S. Bala (USRA) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 05:24:28.61 UT on 21 December 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251221A (trigger 787987473/251221225). which was also detected by Swift (Ambrosi et al. 2025, GCN 43166). The Fermi GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight is 96 degrees. The GBM light curve consists of a single peak with a duration (T90) of about 28 s (50-300 keV). The time-averaged spectrum from T0-13.3 to T0+23.6 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.1 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 330 +/- 100 keV. The event fluence (10-1000 keV) in this time interval is (4.2 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1.4 s in the 10-1000 keV band is 2.0 +/- 0.2 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 240 +/- 130 keV, alpha = -1.0 +/- 0.2 and beta = -1.8 +/- 0.2. 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/43174. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...