TITLE: GCN CIRCULAR NUMBER: 21004 SUBJECT: GRB 170409A: Fermi-LAT detection DATE: 17/04/09 15:48:54 GMT FROM: Judith Racusin at GSFC F. Longo (University and INFN, Trieste), J. L. Racusin (NASA/GSFC), and D. Kocesvki (NASA/MSFC)report on behalf of the Fermi-LAT team: At 02:41:59.49 on April 9, 2017, Fermi-LAT detected high-energy emission from GRB 170904A, which was also detected by Fermi-GBM (trigger 513398525/170409112). The best LAT on-ground location is found to be: RA, Dec = 347.53, -7.16 (degrees, J2000) with an error radius of 0.15 deg (90% containment, statistical error only). This was 82 deg from the LAT boresight at the time of the trigger and triggered an autonomous repoint of the spacecraft. The data from the Fermi-LAT show a significant increase in the event rate that is spatially and temporally consistent with the GBM emission. More than 14 photons above 100 MeV and 2 photons above 1 GeV are observed within 1000 seconds. The highest-energy photon is a 10 GeV event which is observed 442 seconds after the GBM trigger. A Swift ToO has been requested for this burst. 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.