TITLE:   GCN CIRCULAR
NUMBER:  36218
SUBJECT: GRB 240421B: Fermi GBM Observation
DATE:    24/04/21 23:05:42 GMT
FROM:    Cori Fletcher at USRA <cfletcher@usra.edu>

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

"At 09:42:41.91 UT on 21 April 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 240421B (trigger 735385366/240421405).
which was also detected by Swift BAT (M. A. Williams et al. 2024, GCN 36208).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 29 degrees.

The GBM light curve consists of multiple peaks with a duration (T90)
of about 12.3 s (50-300 keV). The time-averaged spectrum
from T0+0.003 to T0+18.4 s is best fit by
a Band function with Epeak = 141 +/- 8 keV,
alpha = -0.91 +/- 0.04, and beta = -2.31 +/- 0.09.

The event fluence (10-1000 keV) in this time interval is
(1.28 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+12 s in the 10-1000 keV band is 31.9 +/- 0.5 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/"