TITLE:   GCN CIRCULAR
NUMBER:  45684
SUBJECT: GRB 260921A: Fermi GBM Observation
DATE:    26/09/21 18:45:14 GMT
FROM:    Rushikesh Sonawane  at IISER TVM <rushikesh23@iisertvm.ac.in>

R. Sonawane (IISER, TVM) reports on behalf of the 
Fermi Gamma-ray Burst Monitor Team:

"At 09:29:08.42 UT on 21 September 2026, the Fermi Gamma-ray Burst Monitor (GBM) 
triggered and located GRB 260921A (trigger 811675753/260921395), 
which was also detected by Swift BAT (Page et al. 2026, GCN 45676).
MASTER reported a possible optical counterpart with a candidate 
host galaxy at z=1.3 (Lipunov et al. 2026, GCN 45678).

The Fermi GBM real-time localization (GCN 45674) is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 95 degrees.

The GBM light curve consists of a single emission episode with a duration (T90) 
of about 17.3 s (50-300 keV). The time-averaged spectrum 
from T0-6.3 to T0+13.9 s is best fit by 
a power law function with an exponential high-energy cutoff. 
The power law index is -0.97 +/- 0.02 and the cutoff energy, 
parameterized as Epeak, is 280 +/- 3 keV.

The event fluence (10-1000 keV) in this time interval is 
(9.3 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured 
starting from T0+4.0 s in the 10-1000 keV band is 12.3 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well 
with Epeak = 261 +/- 3 keV, alpha = -0.95 +/- 0.01 and beta = -2.58 +/- 0.05.

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/"