GCN Circular 43346

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

"At 23:21:36.55 UT on 05 January 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260105B (trigger 789348101/260105973).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 259.71, Dec = 1.74 (J2000 degrees, equivalent to
J2000 17h 18m, +1d 44'), with a statistical uncertainty of 2.03 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 68 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 11 s (50-300 keV). The time-averaged spectrum
from T0-1.9 to T0+12.5 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.96 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 232 +/- 2 keV.

The event fluence (10-1000 keV) in this time interval is
(8 +/- 0.4)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+2.4 s in the 10-1000 keV band is 156 +/- 2 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 178 +/- 2 keV, alpha = -0.83 +/- 0.01 and beta = -2.19 +/- 0.03.


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/"