TITLE: GCN CIRCULAR NUMBER: 38695 SUBJECT: GRB 241228A: Fermi GBM Observation DATE: 24/12/28 21:20:27 GMT FROM: Lorenzo Scotton at UAH lscottongcn@outlook.com

L. Scotton (UAH) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 03:57:10.17 UT on 28 December 2024, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 241228A (trigger 757051035/241228165), which was also detected by Swift BAT (V. D'Elia et al. 2024, GCN 38681). The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 55 degrees.

The GBM light curve consists of a minor triggering episode followed by a two-peak episode, with a duration (T90) of about 8.3 s (50-300 keV). The time-averaged spectrum from T0-0.5 to T0+10.9 s is best fit by a Band function with Epeak = 66 +/- 3 keV, alpha = -0.68 +/- 0.08, and beta = -2.8 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is (5.6 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+7.0 s in the 10-1000 keV band is 19.9 +/- 0.4 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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