GCN Circular 45010

P. Veres (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 16:55:04.71 UT on 18 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260618B (trigger 803494509/260618705),
which was also detected by Fermi/LAT (Gupta et al., GCN 44983
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 ) and SVOM/VT (Li et al., GCN 45001
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 ).
The Fermi GBM on-ground location is consistent with the LAT and the SVOM/VT position.

The angle from the Fermi LAT boresight is 16 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 20 s (50-300 keV). The time-averaged spectrum
from T0 to T0+10 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.42 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 1030 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is
(9.4 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.96 s in the 10-1000 keV band is 4.3 +/- 0.2 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 1000 +/- 20 keV, alpha = -0.41 +/- 0.02 and beta = -3.2 +/- 0.2.

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/"