TITLE:   GCN CIRCULAR
NUMBER:  39823
SUBJECT: GRB 250320B: Fermi GBM Observation
DATE:    25/03/21 21:09:51 GMT
FROM:    eliza.neights@gmail.com

E. Neights (GWU, NASA/GSFC), R. Hamburg (UAH), C. Meegan (UAH) and O.J. Roberts (USRA) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 23:15:22.02 UT on 20 March 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250320B (trigger 764205327/250320969).
which was also detected by AstroSat CZTI (Joshi et al. 2025, GCN 39808), SVOM/GRM (Zhang et al. 2025, GCN 39813), and Fermi LAT (Holzmann Airasca et al. 2025, GCN 39819).
The Fermi GBM on-ground location is consistent with the LAT position.

The angle from the Fermi LAT boresight is 40 degrees.

The GBM light curve consists of multiple emission episodes with a duration (T90)
of about 86 s (50-300 keV). The time-averaged spectrum
from T0-2.0 to T0+107.5 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.16 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 570 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is
(9.02 +/- 0.07)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+75 s in the 10-1000 keV band is 21.6 +/- 0.4 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak = 547 +/- 22 keV, alpha = -1.16 +/- 0.01 and beta = -2.7 +/- 0.2.

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/"