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." View this GCN Circular online at https://gcn.nasa.gov/circulars/42509. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...