GCN Circular 41477

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

At 16:07:47.46 UT on 21 August 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250821B (trigger 777485272/250821672).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 39.95, Dec = -77.13 (J2000 degrees, equivalent to
J2000 2h 39m, -77d 7'), 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 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 67 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 2.3 s (50-300 keV). The time-averaged spectrum
from T0-0.06 to T0+2.62 s is best fit by
a Band function with Epeak = 250 +/- 10 keV,
alpha = -0.67 +/- 0.03, and beta = -2.24 +/- 0.06.

The event fluence (10-1000 keV) in this time interval is
(1.19 +/- 0.01)E-05 erg/cm^2. The 0.064-sec peak photon flux measured
starting from T0+0.576 s in the 10-1000 keV band is 58 +/- 1.9 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/