TITLE:   GCN CIRCULAR
NUMBER:  29602
SUBJECT: GRB 210306A: Fermi-LAT detection
DATE:    21/03/06 14:19:02 GMT
FROM:    Milena Crnogorcevic at U.of Maryland/NASA-GSFC  <milenaGCN@gmail.com>

M. Ohno (Hiroshima Univ. & Eotvos Univ.), M. Axelsson (KTH &
StockholmUniv.), S. Cutini (INFN Perugia), F. Longo (University & INFN
Trieste), and M. Crnogorcevic (Univ. of Maryland & NASA/GSFC)
report on behalf of the Fermi-LAT Collaboration:

On March 06, 2021, Fermi-LAT detected high-energy emission from GRB
210306A, which was also detected by
Fermi-GBM (trigger 636695642), Swift (D'Elia et al.; GCN 29597), and
confirmed the afterglow by ground telescopes
(M. Watson et al.; GCN 29598, Dmitrievich Romanov; GCN 29599,
Strausbaugh and. Cucchiara; GCN 29600).

The best LAT on-ground location is found to be RA, Dec 130.3, 60.2
(degrees, J2000)
with an error radius of 0.38 deg (90% containment, statistical error only).
This was 10 deg from the LAT boresight at the time of the GBM trigger:

T0 =  03:53:57.11 UT.

The data from the Fermi-LAT show a significant increase
in the event rate after the GBM trigger that is spatially correlated with the
GBM emission (5.8 degrees from the GBM location) with high significance.
The photon flux above 100 MeV in the time interval 0-3000 s after the
GBM trigger is (6.6+/-3.5 e-7) ph/cm2/s.

The estimated photon index above 100 MeV is -2.0(+/-0.4).

The highest-energy photon is a 1.9 GeV event which is observed 790 seconds
after the GBM trigger.

The Fermi-LAT point of contact for this burst is
Milena Crnogorcevic (mcrnogor@astro.umd.edu).

The Fermi-LAT is a pair conversion telescope designed to cover
the energy band from 20 MeV to greater than 300 GeV.
It is the product of an international collaboration between
NASA and DOE in the U.S. and many scientific institutions
across France, Italy, Japan and Sweden.