GCN Circular 38903

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

"At 23:14:12.65 UT on 01 January 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250101B (trigger 757466057/250101968).
which was also detected by SVOM/GRM (Zhang et al. 2025, GCN 38793
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 ).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 70.87, Dec = -3.25 (J2000 degrees, equivalent to
J2000 4h 43m, -3d 15'), with a statistical uncertainty of 3.29 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a
systematic error which we have characterized as a mixture of two Gaussians,
one with a radius of 1.8 degrees (52% contribution) and one with a radius
of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).

The angle from the Fermi LAT boresight is 49 degrees.

The GBM light curve consists of a single emission episode with two distinct peaks with a duration (T90)
of about 4.5 s (50-300 keV). The time-averaged spectrum
from T0-1.0 to T0+5.1 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.34 +/- 0.09 and the cutoff energy,
parameterized as Epeak, is 146 +/- 7 keV.

The event fluence (10-1000 keV) in this time interval is
(2.418 +/- 0.8)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.77 s in the 10-1000 keV band is 5 +/- 0.2 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/"