TITLE:   GCN CIRCULAR
NUMBER:  29361
SUBJECT: GRB 210120A: Fermi GBM detection
DATE:    21/01/27 01:39:51 GMT
FROM:    Stephen Lesage at Fermi-GBM Team  <sjl0014@uah.edu>

S. Lesage (UAH), R. Dunwoody (UCD), J. Mangan (UCD), C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 07:10:44.70 UT on 20 January 2021, the Fermi Gamma-Ray Burst Monitor
(GBM)
triggered and located GRB 210120A (trigger 632819449 / 210120.299),
the optical counterpart for which was detected by MASTER (Lipunov et al.
2021,
GCN 29339) and confirmed (Pozanenko et al. 2021, GCN 29341; Dimple et al.
2021,
GCN 29345). The GBM Fermi GBM Final Real-time Localization (GCN 29334) is
consistent
with the MASTER OT follow-up observations.

The angle from the Fermi LAT boresight at the GBM trigger time is 82
degrees.

The GBM light curve consists of a single peak
with a duration (T90) of about 22.2 s (50-300 keV).
The time-averaged spectrum from T0-0.32 s to T0+22.21 s is
best fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.62 +/- 0.12 and
the cutoff energy, parameterized as Epeak, is 94 +/- 6 keV

The event fluence (10-1000 keV) in this time interval is
(2.9 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+ 2.6 s in the 10-1000 keV band
is 3.6 +/- 0.2 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 95 +/- 6 keV, alpha = -0.63 +/- 0.12 and beta = -29 +/- 6E12.

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/"