TITLE: GCN CIRCULAR NUMBER: 41411 SUBJECT: GRB 240818A: Fermi GBM Observation DATE: 25/08/18 15:01:48 GMT FROM: Cuán de Barra at UCD cuan.debarra@ucdconnect.ie
C. de Barra (University College Dublin) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 01:31:23.00 UT on 18 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818A (trigger 777173487/250818063) which was also detected by Swift BAT (S. B. Cenko et al. 2025, GCN 41403). The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 146 degrees.
The GBM light curve consists of multiple emission episodes with a duration (T90) of about 101 s (50-300 keV). The time-averaged spectrum from T0-9.2 to T0+94.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.83 +/- 0.14 and the cutoff energy, parameterized as Epeak, is 83 +/- 5 keV.
The event fluence (10-1000 keV) in this time interval is (1.62 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+78 s in the 10-1000 keV band is 8.8 +/- 0.7 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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