TITLE:   GCN CIRCULAR
NUMBER:  42509
SUBJECT: GRB 251028B: Fermi/GBM observation
DATE:    25/10/29 11:59:34 GMT
FROM:    Christian Malacaria at INAF-OAR <cmalacaria.astro@gmail.com>

C. Malacaria (INAF-OAR) report 
on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 07:48:06.71 UT on 28 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251028B (trigger 783330491/251028325),
which was also detected by the Glowbug (Woolf et al. 2025, GCN 42507) 
and Swift/BAT-GUANO (DeLaunay et al. 2025, GCN 42508).
A final Fermi/GBM localization correction was issued in GCN 42502.

The GBM light curve consists of two main pulses with a duration (T90)
of about 14 s (50-300 keV). The time-averaged spectrum from T0-0.3 s to T0+12.0 s 
is best fit by a power law function with an exponential high-energy cutoff.
The power law index is -0.77 +/- 0.10 and the cutoff energy, parameterized as 
Epeak, is 125 +/- 8 keV.

The event fluence (10-1000 keV) in this time interval is
(3.9 +/- 0.2)E-06 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+8.9 s in the 10-1000 keV band
is 6.4 +/- 0.4 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak = 113 +/- 11 keV, 
alpha = -0.68 +/- 0.14 and beta = -2.8 +/- 0.4.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."