TITLE: GCN CIRCULAR NUMBER: 13777 SUBJECT: GRB 120916A: Fermi-LAT detection of a burst DATE: 12/09/18 02:41:43 GMT FROM: Masanori Ohno at ISAS/JAXA 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.