TITLE: GCN CIRCULAR NUMBER: 10978 SUBJECT: GRB 100724B: Fermi LAT and GBM analysis DATE: 10/07/25 04:32:20 GMT FROM: James Chiang at SLAC Yasuyuki Tanaka, Masanori Ohno (ISAS/JAXA), Hiromitsu Takahashi, Takeshi Uehara (Hiroshima University), Nicola Omodei (Stanford Univ.), James Chiang (SLAC) and Sylvain Guiriec (UAH) report on behalf of the Fermi LAT and GBM teams: At 00:42:06 UT on 24 July 2010, the Fermi Large Area Telescope (LAT) detected gamma-rays from the long GRB 100724B. This burst was detected and localized by the Fermi Gamma-Ray Burst Monitor (GBM) (trigger 301624927 / 100724029, GCN 10977). The best LAT on-ground localization for this burst is RA, Dec = 120.04, 76.74 (08h 00m 09.6s, 76d 44' 24", J2000) with a 90% containment radius of 1.1 degrees (statistical; 68% containment radius 0.6 degrees). At the GBM trigger time, this location was at angle of 49 degrees from the LAT boresight and approximately 18 degrees from the Earth's limb. The GBM trigger caused an Autonomous Repoint Request. However, owing to the proximity of the burst location to the Earth horizon and constraints on the spacecraft pointing, the angle of the source with respect to the boresight remained greater than 40 degrees for the first 2700 seconds after trigger. Using a non-standard data selection designed to maximize the low energy acceptance, the LAT light curve shows two distinct peaks at ~20 and ~64 s after the GBM trigger and a smaller sub-peak at ~77 s post-trigger. A joint spectral fit with the GBM data yields a photon index in the LAT band (> 20 MeV) of -2.48 +/- 0.01 (stat). This is consistent with an extrapolation of the high energy part of the spectrum in the GBM band. This result differs somewhat from the fit reported in GCN 10977. In order to account for a spectral feature below 100 keV, we have included a low energy spectral component in addition to the usual Band function model; this accounts for the difference found in the high energy photon index. Further analysis is ongoing. The Fermi LAT point of contact for this burst is Yasuyuki Tanaka (tanaka@astro.isas.jaxa.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. [GCN OPS NOTE(24aug10): Per author's request, the date in the first line was changed from "25 July" to "24 July".]