GCN Circular 42509
Subject
GRB 251028B: Fermi/GBM observation
Event
Date
2025-10-29T11:59:34Z (17 hours ago)
From
Christian Malacaria at INAF-OAR <cmalacaria.astro@gmail.com>
Via
Web form
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."