GCN Circular 13777

G. Vianello (SLAC), M. Ohno (Hiroshima U.), and V. Connaughton (UAH)  
report on behalf of the Fermi-LAT team:

On Sept. 16, 2012, Fermi LAT detected high energy emission from GRB  
120916A, which was also detected by Fermi-GBM (trigger 120916173).  
Fermi GBM triggered at T0 = 04:08:40 UTC, although emission from the  
GRB is clearly seen starting at ~T0-60 s. GBM did not trigger on this  
earlier emission period owing to the triggering being disabled while  
Fermi was passing through a region of high geomagnetic latitude. The  
GRB was also detected by the IPN which provided us a preliminary  
position (private communication, circular to appear).

The best LAT on-ground location is found to be RA, Dec 205.81, 36.66  
(J2000) with an error radius of 0.3 deg (68% containment, statistical  
error only), this was 29 deg from the LAT boresight at the time of the  
trigger. This localization has been found using LAT SOURCE data class  
in a time interval of ~500 seconds around the GRB time.

The LAT data show a significant increase in counts at energies >100  
MeV, starting around T0, i.e., several seconds after the beginning of  
the low-energy emission. A standard likelihood analysis yields a  
photon index of -2.0 +/- 0.5 and a flux of (1.6 +/- 1.0) x 10^-9 erg/ 
cm^2/s (100 MeV - 10 GeV).

We note that the transient is not significantly detected using LAT  
TRANSIENT class data, even though the LAT TRANSIENT data appear by eye  
to follow the lightcurve seen at lower energies in the GBM. This lack  
of a significant signal in the LAT TRANSIENT data is due to the low  
signal-to-noise ratio in that class resulting from the high non-photon  
background corresponding to the high geomagnetic latitude.

The Fermi LAT point of contact for this burst is Masanori Ohno (ohno@hep01.hepl.hiroshima-u.ac.jp 
).

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.