TITLE:   GCN CIRCULAR
NUMBER:  30225
SUBJECT: GRB 210421B: Fermi GBM detection
DATE:    21/06/12 17:55:20 GMT
FROM:    Joe Mangan at UCD  <joseph.mangan@ucdconnect.ie>

J. Mangan (UCD), R. Dunwoody (UCD), C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

At 10:54:44.812 UT on 21 Apr 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 210421B (trigger 640695289 / 210421455)
which was also detected by the Swift/BAT-GUANO (DeLaunay et al. 2021, GCN
29877)
The Fermi GBM Final Real-time Localization (GCN 29870) is consistent with
the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 20.1
degrees.

The GBM light curve consists of a single peak
with a duration (T90) of about 11 s (50-300 keV).
The time-averaged spectrum from T0-1.536 s to T0+9.728 is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.98 +/- 0.11 and
the cutoff energy, parameterized as Epeak, is 1253.00 +/- 500.00 keV

The event fluence (10-1000 keV) in this time interval is
(3.655 +/- 0.225)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-0.384 s in the 10-1000 keV band
is 2.74 +/- 0.26 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/