TITLE: GCN CIRCULAR NUMBER: 38712 SUBJECT: GRB 241229B: Fermi GBM Observation DATE: 24/12/29 18:21:40 GMT FROM: Peter Veres at University of Alabama in Huntsville veresp@gmail.com

Peter Veres (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 01:36:16.06 UT on 29 December 2024, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 241229B (trigger 757128981/241229067). which was also detected by Swift BAT (H. A. Krimm et al. 2024, GCN 38699). The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 39 degrees.

The GBM light curve consists of a single emission episode with a duration (T90) of about 6 s (50-300 keV). The time-averaged spectrum from T0-5.1 to T0+4.1 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.7 +/- 0.2 and the cutoff energy, parameterized as Epeak, is 76 +/- 5 keV.

The event fluence (10-1000 keV) in this time interval is (1.24 +/- 0.06)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.064 s in the 10-1000 keV band is 6.1 +/- 0.3 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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