TITLE: GCN CIRCULAR NUMBER: 19413 SUBJECT: GRB 160509A: Fermi-LAT refined analysis DATE: 16/05/09 18:09:47 GMT FROM: Elisabetta Bissaldi at U.Innsbruk/IAPP F. Longo (University and INFN, Trieste), E. Bissaldi (Politecnico and INFN, Bari), G. Vianello (Stanford U.), E. Moretti (MPI, Munich), N. Omodei (Stanford U.), J. Bregeon (CNRS/IN2P3/LUPM), F. Dirirsa (Johannesburg U.), M. Yassine (LUPM, Montpellier), D. Kocevski, J. Racusin, J. McEnery (all NASA/GSFC), M. Ohno (Hiroshima U.) and S. Zhu (AEI Potsdam-Golm / AEI Hannover) report on behalf of the Fermi-LAT Team: Using LAT source class events >100 MeV from 0 until 2660 seconds after the LAT trigger, we find a LAT localization of RA, Dec = 311.3, 76.1 (J2000 deg) with a 90% containment radius of 0.12 degrees (statistical only). The LAT executed an autonomous repoint 2 minutes after the trigger to follow the burst for 2.5 hours. A Fermi ToO for the next 36 hours has been issued. Following our detection, Swift followed the GRB and detected the X-ray afterglow with the Swift/XRT (Kennea et al. GCN 19408). The highest-energy photon is a 52 GeV event, which is observed 77 seconds after the GBM trigger. The LAT Low Energy (LLE) emission consists of two bright structured peaks (around T0+12 and T0+18 s), in coincidence with the main GBM emission episode. The >100 MeV emission spectrum during the main GBM emission episode (from T0 to T0+40 s, see Roberts et al. GCN 19411) is fit by a soft power-law with index -3.4 +/- 0.2, resulting in a flux of (0.56 +/- 0.06)E-4 ph cm^-2 s^-1. The >100 MeV emission spectrum after 40 s and up to the currently available data (up to 2660 s) is fit by a power-law with index -2.0 +/- 0.1. The Fermi LAT point of contact for this burst is Francesco Longo (francesco.longo@ts.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.