TITLE: GCN CIRCULAR NUMBER: 42383 SUBJECT: GRB 251016B: Fermi GBM Detection DATE: 25/10/23 06:22:48 GMT FROM: Matt Godwin msg0028@uah.edu

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/"

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