TITLE: GCN CIRCULAR NUMBER: 19403 SUBJECT: GRB 160509A: Fermi-LAT prompt detection of a very bright burst DATE: 16/05/09 09:42:59 GMT FROM: Elisabetta Bissaldi at U.Innsbruk/IAPP F.Longo (University and INFN, Trieste), E.Bissaldi (Politecnico and INFN, Bari), J. Bregeon (CNRS/IN2P3/LUPM), J. McEnery (NASA/GSFC), M. Ohno (Hiroshima U), S. Zhu (AEI Potsdam-Golm/AEI Hannover) report on behalf of the Fermi-LAT team: At 08:59:04.36 UTC on May 09, 2016, Fermi-LAT triggered on high-energy emission from GRB 160509A, also detected by GBM (trigger 484477130/160509374). The onboard location is RA, Dec 310.1, 76.0 (J2000) with an estimated error radius of 0.50 deg (90% containment, systematic error only). This was 31.8 deg from the LAT boresight at the time of the trigger. We anticipate providing a refined location within the next 12 hours when the LAT science data for this burst is downlinked and processed. We note that this is an extraordinarily bright GRB, and strongly encourage additional observations. 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.