TITLE: GCN CIRCULAR NUMBER: 40829 SUBJECT: GRB 250620B: Fermi GBM Observation DATE: 25/06/25 23:57:52 GMT FROM: oindabimukherjee@gmail.com
O. Mukherjee (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 13:22:56.94 UT on 20 June 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB250620B (trigger 772118581/250620558). which was also detected by Swift/BAT-GUANO (S. Ronchini et al. 2025, GCN 40800). The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.
The angle from the Fermi LAT boresight is 90 degrees.
The GBM light curve consists of a single emission episode with a duration (T90) of about 23 s (50-300 keV). The time-averaged spectrum from T0-2 to T0+23.6 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.3 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 430 +/- 236 keV.
The event fluence (10-1000 keV) in this time interval is (2.7 +/- 0.4)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.7 s in the 10-1000 keV band is 1.9 +/- 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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