GCN Circular 36535

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

"At 15:08:57.06 UT on 21 May 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 240521B (trigger 737996942/240521631).
which was also detected by Swift/BAT-GUANO (Ronchini et al. 2024, GCN 36532
Not found
 
 
).
The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.

The angle from the Fermi LAT boresight is 101 degrees.

The GBM light curve single emission episode with a duration (T90)
of about 34 s (50-300 keV). The time-averaged spectrum
from T0-16 to T0+17 s is best fit by
a simple power law function with index -1.64 +/- 0.04.

The event fluence (10-1000 keV) in this time interval is
(4 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-9.9 s in the 10-1000 keV band is 4.1 +/- 0.4 ph/s/cm^2.

A power law function with an exponential high-energy cutoff fits the spectrum equally well. The power law index is -1.48 +/- 0.11 and the cutoff energy,
parameterized as Epeak, is 222 +/- 94 keV.

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/"