GCN Circular 30253

J. Wood (NASA/MSFC) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 01:43:36.14 UT on 18 June 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210618A (trigger 645673421 / 210618072)
which was also detected by the Swift/BAT (M. G. Bernardini et al. 2019, GCN 30250).

The Fermi GBM Final Real-time Localization (GCN 30248) is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 113 degrees.

The GBM light curve consists of multiple peaks
with a duration (T90) of about 3.6 s (50-300 keV).
The time-averaged spectrum from T0-1.6 s to T0+2.2 s is
well fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.62 +/- 0.26 and
the cutoff energy, parameterized as Epeak, is 440 +/- 140 keV

The event fluence (10-1000 keV) in this time interval is
(1.2 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.3 s in the 10-1000 keV band
is 2.1 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 400 +/- 190 keV, alpha = -0.60 +/- 0.32 and beta = -2.3 +/- 0.9.

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/"