TITLE: GCN CIRCULAR NUMBER: 12188 SUBJECT: GRB 110721A: Fermi-LAT detection DATE: 11/07/21 15:09:33 GMT FROM: Vlasios Vasileiou at LUPM/Fermi-LAT V. Vasileiou (CNRS/IN2P3/LUPM), F. Piron (CNRS/IN2P3/LUPM), D. Tierney (UCD), A. von Kienlin (MPE), S. Guiriec (UAH), and J. L. Racusin (NASA/GSFC) report on behalf of the Fermi-LAT team: At 04:47:45 on July 21, 2011, Fermi-LAT detected high energy emission from GRB 110721A, which was also detected by Fermi-GBM (trigger 332916465, Tierney et al. (GCN 12187)), and Integral (SPI ACS trigger 6326). The best preliminary LAT on-ground location is found to be (RA, DEC)= (333.4,-39.0) (J2000; deg) with an error radius of 0.75 (0.51) deg at 90% (68%) containment (statistical error only), which was 40 deg from the LAT boresight at the time of the trigger. We further report that the Fermi Observatory executed an autonomous repoint maneuver to follow this trigger for the next 2.5 hours, subject to Earth-angle constraints. The data from the Fermi-LAT show a significant increase in the event rate that is spatially and temporally correlated with the GBM emission with high significance. This detection was independently confirmed by the blind on-ground GRB search of the LAT data. More than 20 (1) photons above 100 MeV (1 GeV) using a standard event selection and about 1000 photons using a non-standard event selection sensitive mostly to tens-of-MeV energies are observed within 20 sec. The lightcurve at tens-of-MeV energies has a Fast Rise Exponential Decay (FRED) profile with an ~16 sec duration similar to the GBM results. The highest energy event has an 1.7 GeV energy, was observed about 0.7 sec after the trigger, and was associated to the GRB with a high (>0.9) probability. Preliminary spectral analyses on the LAT data taken during the first 20 sec after the trigger resulted in a -2.9+-0.4 spectral index and a 3.3+-0.8 x 10^-4 ph/cm^2/sec average flux integrated over E>100 MeV. This spectral index is considerably softer than the extrapolation of the GBM spectral index at E>100 MeV energies (Tierney et al. (GCN 12187)), which suggests a spectral cutoff or softening of the spectrum. A Swift Target of Opportunity observation has been requested and tiling observations are ongoing. The Fermi LAT point of contact for this burst is Vlasios Vasileiou (vlasios.vasileiou@lupm.in2p3.fr). 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.