[vsnet-grb-info 28584] GRB 210822A: Fermi-LAT detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30687
SUBJECT: GRB 210822A: Fermi-LAT detection
DATE:    21/08/22 20:02:07 GMT
FROM:    Masanori Ohno at Hiroshima U  <ohno at astro.hiroshima-u.ac.jp>

M. Ohno (Eotvos U./Hiroshima U.), D. Kocevski (NASA/MSFC), F. Longo
(University and INFN, Trieste), a F. Dirirsa (LAPP, Annecy)
report on behalf of the Fermi-LAT Collaboration:

On August 22th, 2021, Fermi-LAT detected high-energy emission from
GRB 210822A, which was also detected by Swift-BAT (trigger1069788; Page et
al. GCN Circ. 30677) and GECAM (Wang et al., GCN Circ. 30678).


The best LAT on-ground location is found to be

RA, Dec 304.6, 4.9 (degrees, J2000)

with an error radius of 0.5 deg (90% containment, statistical error only).
This was 83 deg from the LAT boresight at the time of the Swift trigger:

T0 = 09:18:18 UT.

And the burst came into the LAT boresight about 500 s after the Swift
trigger time.
The data from the Fermi-LAT show a significant increase
in the event rate after the Swift trigger that is spatially correlated with
the Swift emission with high significance.
The photon flux above 100 MeV in the time interval 500-10000 s after the
Swift trigger is 2.5(-/+ 0.8)  ph/cm2/s.

The estimated photon index above 100 MeV is -2.4 (-/+ 0.3).

The highest-energy photon is a 1.0 GeV event which is observed 855 seconds
after the Swift trigger.

The Fermi-LAT point of contact for this burst is
Feraol F. Dirirsa (dirirsa at lapp.in2p3.fr ).

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.