TITLE:   GCN CIRCULAR
NUMBER:  38714
SUBJECT: GRB 241228B: Fermi GBM Observation
DATE:    24/12/29 21:37:56 GMT
FROM:    Lorenzo Scotton at UAH <lscottongcn@outlook.com>

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

"At 04:13:05.39 UT on 28 December 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 241228B (trigger 757051990/241228176),
which was also detected by Swift/BAT-NITRATES (J. DeLaunay et al. 2024, GCN 38700).
The Fermi GBM on-ground location is consistent with the Swift/BAT-NITRATES position.

Follow-up observations report the detection of the potential afterglow 
(Kumar et al. GCN 38684 and GCN 38691; An et al. GCN 38687; Ortega-Casas et al. GCN 38692; 
Mohan et al. GCN 38694; Ghosh et al. GCN 38702, Burrows et al. GCN 38713) 
at a redshift z = 2.674 (An et al. GCN 38704).

The angle from the Fermi LAT boresight is 49 degrees.

The GBM light curve consists of two emission episodes, with a duration (T90)
of about 19.5 s (50-300 keV). The time-averaged spectrum
from T0-0.7 to T0+23.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.86 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 440 +/- 10 keV.

The event fluence (10-1000 keV) in this time interval is
(4.66 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.2 s in the 10-1000 keV band is 15.1 +/- 0.3 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/"