TITLE:   GCN CIRCULAR
NUMBER:  31659
SUBJECT: GRB 220228A: Fermi-LAT detection
DATE:    22/02/28 22:06:43 GMT
FROM:    Niccolo Di Lalla at Stanford U  <niccolo.dilalla@stanford.edu>

N. Omodei, N. Di Lalla (Stanford University), R. Pillera (INFN Bari) 
report on behalf of the Fermi-LAT team:

On February 28, 2022, Fermi-LAT detected high-energy emission from GRB 
220228A, which was also detected by**Fermi-GBM (trigger 667737010 / 
220228438, GCN 31658).

The best LAT on-ground location is found to be

RA, Dec = 311.2, -22.7 (degrees, J2000)

with an error radius of 0.7 deg (90 % containment, statistical error 
only). This was 23 deg from the LAT boresight at the time of the trigger:

T0 = 10:30:05.92 UT.

The data from the Fermi-LAT show a significant increase in the event 
rate that is spatially and temporally correlated with the GBM emission 
with high significance. The photon flux above 90 MeV in the time 
interval 0-1000s after the GBM trigger is 4E-6 ± 1E-6 ph/cm2/s.

The estimated photon index above 90 MeV is -2.9 ± 0.4.

The highest-energy photon is a 323 MeV**event which is observed 77 
seconds after the GBM trigger.

A Swift ToO has been requested for this burst.

The Fermi-LAT point of contact for this burst is Roberta 
Pillera**(roberta.pillera@ba.infn.it).

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.