TITLE: GCN CIRCULAR NUMBER: 32851 SUBJECT: GRB 221025A: Fermi GBM Observation DATE: 22/10/25 16:52:20 GMT FROM: Joshua Wood at MSFC/Fermi-GBM J. Wood (NASA/MSFC) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 00:58:18 UT on 25 October 2022, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 221025A (trigger 688352303 / 221025040). which was also detected by Fermi-LAT (Arimoto M. et al. 2022, GCN 32843). The Fermi GBM on-ground location is consistent with the Fermi-LAT position. The on-ground calculated location, using the GBM trigger data, is RA = 128.9, DEC = +2.2 (J2000 degrees, equivalent to 08h 36m, +02d 12'), with an uncertainty of 6.8 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a core-plus-tail model, with 90% of GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg systematic error. [Connaughton et al. 2015, ApJS, 216, 32]). The angle from the Fermi LAT boresight is 48.5 degrees. The GBM light curve consists of a single pulse with a duration (T90) of about 0.4 s (50-300 keV). The time-averaged spectrum from T0+0.0 to T0+0.8 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.4 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 1680 +/- 345 keV. The event fluence (10-1000 keV) in this time interval is (1.7 +/- 0.1)E-06 erg/cm^2. The 64-ms peak photon flux measured starting from T0+0.3 s in the 10-1000 keV band is 13.3 +/- 0.9 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/"