GCN 43489: GRB 260120B: Fermi GBM Observation

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

"At 18:00:26.05 UT on 20 January 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260120B (trigger 790624831/260120750),
which was also detected by Swift BAT (Ferro et al. 2026, GCN 43479), Swift XRT (Goad et al. 2026, GCN 43484),
 and SVOM/C-GFT (Wu et al. 2026, GCN 43483).
The Fermi GBM real-time localization (GCN 43477) is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 10 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 7 s (50-300 keV). The time-averaged spectrum
from T0-1.1 to T0+9.9 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.19 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 214 +/- 4 keV.

The event fluence (10-1000 keV) in this time interval is
(3.1 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.38 s in the 10-1000 keV band is 5.8 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 152 +/- 2 keV, alpha = 0.36 +/- 0.02 and beta = -2.26 +/- 0.05.

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/"