TITLE: GCN CIRCULAR NUMBER: 10163 SUBJECT: GRB 091031: Fermi LAT detection of a faint burst DATE: 09/11/12 22:41:27 GMT FROM: Francesco de Palma at U of INFN Bari F. de Palma (University and INFN Bari), N. Omodei (INFN Pisa), Julie McEnery (NASA/GSFC) and V. Vasileiou (NASA GSFC/UMBC) report on behalf of the Fermi LAT team: At 12:00:28 (UT) on 31 October 2009 , the Fermi Large Area Telescope (LAT) detected gamma rays from the long GRB 091031, which was triggered and located by the Fermi Gamma-ray Burst Monitor (GBM) (trigger 278683230 / 091031500,GCN 10115). The angle of the GBM best position (RA, Dec= 70.58, -59.08) with respect to the LAT boresight was ~22 degrees at the time of the trigger, which is well inside our field of view. The Fermi LAT events after the GBM trigger are spatially and temporally correlated with the emission measured by GBM. In the time interval T0-T0+100s using likelihood methods on photons above 100 MeV this burst has a significance of more than 3 sigma; using counting methods the significance is well over 6 sigma in the full energy band (20 MeV - 300 GeV). More than 30 photons above 100 Mev and 2 photons above 1 GeV were observed in the same time interval in an energy dependent ROI centered on the GBM position. The best LAT on-ground localization is found to be (RA, Dec = 71.7, -57.5) with a 90% containment radius of 0.3 deg (statistical; 68% containment radius: 0.2 deg, preliminary systematic error is less than 0.1 deg) which is consistent with the GBM localization. Further analysis is ongoing. The point of contact for this burst is Francesco de Palma : francesco.depalma@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. This message can be cited.