GCN Circular 29140

R. Hamburg (UAH), C. Malacaria (NASA-MSFC/USRA), and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 23:06:34.33 UT on 21 December 2020, the Fermi Gamma-Ray Burst Monitor
(GBM)
triggered and located GRB 201221D (trigger 630284799 / 201221963)
which was also detected by the Swift/BAT and Swift/XRT
(Page et al. 2020, GCN 29112
Not found
 
 
) and Konus-Wind (Frederiks et al. 2020,
GCN 29130
Not found
 
 
). This trigger was initially classified as a  particle event by
the flight software, but is in fact a GRB. The GBM on-ground location is
consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 89
degrees.

The GBM light curve shows a single-peaked structure
with a duration (T90) of about 0.14 s (50-300 keV).
The time-averaged spectrum from T0-0.064 s to T0+0.192 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.20 +/- 0.16 and
the cutoff energy, parameterized as Epeak, is 108 +/- 5 keV.
A Band function fits the spectrum equally well with
Epeak = 98 +/- 8 keV, alpha = 0.01 +/- 0.24 and beta = -3.3 +/- 0.5.

The event fluence (10-1000 keV) in this time interval is
(1.076 +/- 0.046)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 41 +/- 2 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/"