TITLE: GCN CIRCULAR NUMBER: 10070 SUBJECT: GRB 091024: Fermi GBM Observations DATE: 09/10/24 17:12:37 GMT FROM: Valerie Connaughton at MSFC Elisabetta Bissaldi (MPE) and Valerie Connaughton (UAH) report on behalf of the Fermi GBM Team: "At 08:55:58.47 UT on 24 October 2009, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 091024 (trigger 278067360 / 091024372), which was also detected by the Swift-BAT (Marshall et al. 2009, GCN 10062). The GBM on-ground location is consistent with the Swift position. At 09:06:29.36, GBM triggered on what appears to be the continuation of this burst (trigger 278067991 / 091024380). The GBM on-ground location for this second trigger is also consistent with the Swift position for GRB 091024. The angle from the Fermi LAT boresight at the time of the first trigger is 97 degrees and at the time of the second trigger it is 14 degrees. Furthermore, at 09:12:14.28, the Fermi spacecraft executed a maneuver to place the burst near the center of the Large Area Telescope (LAT) field-of-view and observe this region for 5 hours subject to Earth angle constraints. The first emission episode appears to last about 50 s, with 2 peaks separated by a few seconds. A brighter emission periods starts at the time of the 2nd trigger, 630 s later, and persists for at least 400 s. This second emission period shows an initial pulse (at 630 s) lasting about 40 s, followed by a multi-peaked episode starting 210 s later (840 s from the first trigger) and lasting over 100 s. There is evidence for lower-level emission beyond this time. The Fermi spacecraft entered the South Atlantic Anomaly 2200 s after trigger 091024380, by which time there was no obvious emission in GBM. In a first, preliminary analysis, we have fit time-integrated spectra for each of the 3 main emission blocks. The first trigger appears to have an EPeak of about 400 keV; the first peak of the second trigger is quite weak and best fit using a power-law of index around -1.5; the long and brighter emission period of the second trigger has an EPeak of around 300 keV. Detailed spectral analyses and fluence values will be given in a future circular. The POC for this burst is Elisabetta Bissaldi (ebs@mpe.mpg.de).