GCN Circular 43174

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
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 ).
 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/"