GCN Circular 42383

Matt Godwin (UAH) and C. Meegan (UAH) report on behalf of
 the Fermi Gamma-ray Burst Monitor Team:
 
 "At 23:59:00.72 UT on 16 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
 triggered and located GRB 251016B (trigger 782351945/251016999).
 
 The on-ground calculated location, using the Fermi GBM trigger data,
 is RA = 183.64, Dec = 59.60 (J2000 degrees, equivalent to
 J2000 12h 14m, +59d 36'), with a statistical uncertainty of 1.00 degrees.
 (radius, 1-sigma containment, statistical only; there is additionally a
 systematic error which we have characterized as a mixture of two Gaussians,
 one with a radius of 1.8 degrees (52% contribution) and one with a radius
 of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).
 
 The angle from the Fermi LAT boresight is 46 degrees.
 
 The GBM light curve consists of a single bright emission episode with a duration (T90)
 of about 7.1 s (50-300 keV). The time-averaged spectrum
 from T0-0.8 to T0+13.5 s is best fit by
 a power law function with an exponential high-energy cutoff.
 The power law index is -1.07 +/- 0.01 and the cutoff energy,
 parameterized as Epeak, is 310 +/- 1 keV.
 
 The event fluence (10-1000 keV) in this time interval is
 (2.27 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
 starting from T0+2.4 s in the 10-1000 keV band is 37.5 +/- 0.5 ph/s/cm^2.
 
 A Band function fits the spectrum equally well
 with Epeak= 222 +/- 1 keV, alpha = -0.96 +/- 0 and beta = -2 +/- 0.01.
 
 
 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/"