TITLE: GCN CIRCULAR NUMBER: 43068 SUBJECT: GRB 251209A: Fermi GBM observation DATE: 25/12/10 22:23:56 GMT FROM: Elisabetta Bissaldi at Politecnico and INFN Bari elisabetta.bissaldi@ba.infn.it

E. Bissaldi (Politecnico and INFN Bari) reports on behalf of the Fermi GBM Team:

"At 13:24:03.99 UT on 9 December 2025, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 251209A (trigger 786979448 / 251209558). The Fermi GBM Final Real-time Localization is reported in GCN 43048.

The angle from the Fermi LAT boresight at the GBM trigger time is 52 degrees.

The GBM light curve consists of several structured emission episodes with a duration (T90) of about 57 s (50-300 keV). The time-averaged spectrum from T0-4 s to T0+81 s is best fit by a Band function with Epeak = 105 +/- 4 keV, alpha = -1.63 +/- 0.01, and beta = -2.37 +/- 0.06.

The event fluence (10-1000 keV) in this time interval is (6.89 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+49.7 s in the 10-1000 keV band is 38.4 +/- 0.6 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/"

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