GCN Circular 41876

Matt Godwin (UAH) and C. Meegan (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 13:29:21.01 UT on 16 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250916A (trigger 779722166/250916562).
which was also detected by AstroSat ( Arya et al. 2025, GCN 41843
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 ) with an optical counterpart by GOTO (Belkin et al. 2025, GCN 41847
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 ).

The angle from the Fermi LAT boresight is 66 degrees.

GBM triggered on what appears to be precursor and the main emission episode begins about 150 seconds after the precursor. The lightcurve has a duration (T90)
of about 80 s (50-300 keV). The time-averaged spectrum of the main emission
from T0+230 to T0+290 s is best fit by
a Band function with Epeak = 144 +/- 9 keV,
alpha = -1.08 +/- 0.03, and beta = -2.00 +/- 0.03.

The event fluence (10-1000 keV) within this time interval is
(4.61 +/- 0.07)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+270 s in the 10-1000 keV band is 5.65 +/- 0.4 ph/s/cm^2.

The time-averaged spectrum of the percursor
from T0-12 to T0+05 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.92 +/- 0.3 and the cutoff energy,
parameterized as Epeak, is 110 +/- 22 keV.

The event fluence (10-1000 keV) within this time interval is
(1.31 +/- 0.15)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+270 s in the 10-1000 keV band is 0.67 +/- 0.05 ph/s/cm^2. The localization for the preliminary pulse is uncertain,
but consistent with the secondary pulse.

The event fluence (10-1000 keV) within the whole time interval is
(4.73 +/- 0.16)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+270 s in the 10-1000 keV band is 4.57 +/- 0.3 ph/s/cm^2. The localization for the preliminary pulse is uncertain,
but consistent with the secondary pulse.

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/"