TITLE: GCN CIRCULAR NUMBER: 38502 SUBJECT: GRB 241209A: Fermi GBM Observation DATE: 24/12/09 21:39:39 GMT FROM: rhamburg@usra.edu
R. Hamburg (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 05:35:44.16 UT on 09 December 2024, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 241209A (trigger 755415349/241209233), which was also detected by Swift BAT (Parsotan et al. 2024, GCN 38476) and Swift XRT (Evans et al. 2024, GCN 38486). The Fermi GBM on-ground location is consistent with the Swift position. VLT has also performed optical observations of GRB 241209A (de Ugarte Postigo et al., GCN 38484) and measured a potential redshift of z = 1.490 (Saccardi et al. 2024, GCN 38487).
The angle from the Fermi LAT boresight is 74 degrees.
The GBM light curve consists of a single peak with a duration (T90) of about 0.1 s (50-300 keV). The time-averaged spectrum from T0-0.096 to T0+0.128 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.02 +/- 0.44 and the cutoff energy, parameterized as Epeak, is 140 +/- 25 keV.
The event fluence (10-1000 keV) in this time interval is (1.2 +/- 0.2)E-07 erg/cm^2. The 64-ms peak photon flux measured starting from T0-0.064 s in the 10-1000 keV band is 5.3 +/- 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/"
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