TITLE: GCN CIRCULAR NUMBER: 40766 SUBJECT: GRB 250617B: Fermi GBM Observation DATE: 25/06/18 04:32:42 GMT FROM: sumanbala2210@gmail.com S. Bala (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 21:01:28.19 UT on 17 June 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250617B (trigger 771886893/250617876). which was also detected by Swift BAT (Page et al. 2025, GCN 40760). The Fermi GBM on-ground location is consistent with the Swift BAT position. The angle from the Fermi LAT boresight is 91 degrees. The GBM light curve consists of two distinct emission episodes with a duration (T90) of about 32 s (50-300 keV). The time-averaged spectrum from T0-4.6 to T0+32.3 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.2 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 40 +/- 2 keV. The event fluence (10-1000 keV) in this time interval is (3.4 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+23 s in the 10-1000 keV band is 6.9 +/- 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/" View this GCN Circular online at https://gcn.nasa.gov/circulars/40766. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...