GCN Circular 44972

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

"At 21:36:43.68 UT on 17 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260617A (trigger 803425008/260617900).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 224.34, Dec = -12.61 (J2000 degrees, equivalent to
J2000 15h 47m, -16d 43'), with a statistical uncertainty of 7.08 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a
systematic error which we have characterized as a mixture of two Gaussians,
one with a radius of 1.8 degrees (52% contribution) and one with a radius
of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).

The angle from the Fermi LAT boresight is 99 degrees.

The GBM light curve consists of a short, bright burst with a duration (T90)
of about 0.14 s (50-300 keV). The time-averaged spectrum
from T0-0.08 to T0+0.259 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.56 +/- 0.04 and the cutoff energy,
parameterized as Epeak, is 700 +/- 40 keV.

The event fluence (10-1000 keV) in this time interval is
(7.4 +/- 0.5)E-07 erg/cm^2. The 64-ms peak photon flux measured
starting from T0+0.0 s in the 10-1000 keV band is 21 +/- 1 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/"