TITLE: GCN CIRCULAR NUMBER: 38524 SUBJECT: GRB 241209C: Fermi GBM Observation DATE: 24/12/10 14:15:29 GMT FROM: Christian Malacaria at INAF-OAR cmalacaria.astro@gmail.com

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

"At 07:22:54.60 UT on 09 December 2024, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 241209C (trigger 755421779/241209308), which was also detected by AstroSat (J. Joshi et al. 2024, GCN 38490) and Swift/BAT-GUANO (J. DeLaunay et al. 2024, GCN 38501). The GBM Final Real-time Localization is reported in GCN 38482.

The angle from the Fermi LAT boresight is 36 degrees.

The GBM light curve consists of a single emission episode with a duration (T90) of about 11 s (50-300 keV). The time-averaged spectrum from T0-4.1 to T0+9.2 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 270 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is (4 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+4.8 s in the 10-1000 keV band is 3.3 +/- 0.2 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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