TITLE: GCN CIRCULAR NUMBER: 15472 SUBJECT: GRB 131108A: Fermi-LAT refined analysis DATE: 13/11/09 11:04:27 GMT FROM: Giacomo Vianello at SLAC G. Vianello (Stanford), N.Omodei (Stanford), J.Racusin (NASA/GSFC) and M. Ohno (Hiroshima) report on behalf of the Fermi LAT team: We have analyzed 1.5 ks of data for GRB 131108A which triggered onboard Fermi/LAT (Racusin et al, GCN 15464) as well as Fermi/GBM. We confirm the detection of the GRB in our standard analysis at the position of the afterglow candidate (Stroh and Kennea, GCN 15468; Gorosabel et al., GCN 15469; de Ugarte Postigo et al., GCN 15470), with a very large significance (>> 10 sigma). As seen by Fermi/LAT the burst is bright, but relatively soft. In this preliminary analysis the light curve appears well correlated with the emission at lower energy, as seen by Fermi/GBM. More than 118 photons above 100 MeV and more than 3 photons above 1 GeV are observed within 1500 seconds. The highest energy photon is a 1.5 GeV event which is observed 66 seconds after the GBM trigger, which at redshift 2.4 (GCN 15470) corresponds to 5 GeV rest-frame. The time-integrated spectrum in the same time interval can be well described by a power law with photon index -2.66 +/- 0.12, with a mean flux of 1.0e-08 +/- 1.3e-09 erg/cm2/s (100 MeV - 100 GeV). Fermi-LAT pointed observations are ongoing. 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.