GCN Circular 40766

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
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 ).
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/"