TITLE: GCN CIRCULAR NUMBER: 42520 SUBJECT: GRB 251028A: Fermi GBM Observation DATE: 25/10/29 20:08:12 GMT FROM: Peter Veres at University of Alabama in Huntsville veresp@gmail.com

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

"At 03:26:31.86 UT on 28 October 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251028A (trigger 783314796/251028143) which was also detected by MAXI (Negoro et al. 2025, GCN 42498) and Glowbug (Cheung et al. 2025, GCN 42506). The Fermi GBM on-ground location is consistent with the MAXI position.

The angle from the Fermi LAT boresight is 50 degrees.

The GBM light curve consists of multiple overlapping pulses with a duration (T90) of about 30 s (50-300 keV). The time-averaged spectrum from T0 to T0+39.9 s is best fit by a Band function with Epeak = 244 +/- 14 keV, alpha = -0.91 +/- 0.03, and beta = -2.4 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is (3.13 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+24 s in the 10-1000 keV band is 10.8 +/- 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/"

View this GCN Circular online at https://gcn.nasa.gov/circulars/42520. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...