GCN Circular 39642

P. McDermott (UCD), P. Veres (UAH) and L. Scotton (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 07:38:30.66 UT on 09 March 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250309B (trigger 763198715/250309318).
The Fermi GBM on-ground location was reported in GCN 39635
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 .

The angle from the Fermi LAT boresight is 77 degrees.

The GBM light curve consists of an emission episode with 
multiple peaks, with a duration (T90)
of about 6.4 s (50-300 keV). The time-averaged spectrum
from T0+0.003 to T0+8.192 s is best fit by 
a Band function with Epeak = 93 +/- 5 keV, 
alpha = -0.98 +/- 0.06 and beta = -2.40 +/- 0.08.

The event fluence (10-1000 keV) in this time interval is
(1.07 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+3.8 s in the 10-1000 keV band is 23 +/- 0.4 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/"