GCN Circular 45616

A. C. Trigg (NPP ORAU, NASA MSFC) reports report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 15:11:04.34 UT on 16 September 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located
GRB 260916B (trigger 811264269/260916633).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 352.6, Dec = -35.7 
(J2000 degrees, equivalent to J2000 23h 27m, -35d 55'), with a statistical uncertainty of 4.6 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have
characterized as a core-plus-tail model, with 90% of GRBs having a 3.7 deg error and a small tail
suffering a larger than 10 deg systematic error [Connaughton et al. 2015, ApJS, 216, 32]).

The angle from the Fermi LAT boresight is 110 degrees.

The GBM light curve consistent with a short single emission episode with a duration (T90) of about 0.1
s (50-300 keV). The time-averaged spectrum from T0-0.07 to T0+0.23 s is best fit by a power law
function with an exponential high-energy cutoff. The power law index is 0.24 +/- 0.01 and the cutoff
energy, parameterized as Epeak, is 980 +/- 30 keV.

The event fluence (10-1000 keV) in this time interval is (1.06 +/- 0.07)E-06 erg/cm^2. The 64-ms peak
photon flux measured starting from T0+0.00 s in the 10-1000 keV band is 13.4 +/- 1.0 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/"