TITLE: GCN CIRCULAR NUMBER: 9872 SUBJECT: GRB 090902B: Fermi LAT and GBM refined analysis. DATE: 09/09/03 07:36:42 GMT FROM: Valerie Connaughton at MSFC F. de Palma (Università e INFN Bari), E. Bissaldi (MPE), H. Tajima (SLAC), S. Guiriec (UAH), N. Omodei (INFN Pisa), V. Vasileiou (NASA GSFC/UMBC) and V. Connaughton (UAH) report for the Fermi LAT and GBM teams: Further analysis of GRB 090902B (Bissaldi et al. GCN 9866, de Palma et al. GCN 9867, Kennea et al. GCN 9868) reveals it is detected in the Fermi Large Area Telescope (LAT) at least until 300 s after the Fermi Gamma-Ray Burst Monitor (GBM) trigger time, T0=11:05:08.31 UT. Spectral analysis of the main emission episode, from T0 to T0+25 s, shows a deviation from the Band function obtained from fitting GBM data between 50 keV and 40 MeV (GCN 9866). This deviation is apparent both below 50 keV in the GBM and above 100 MeV in the LAT. It is well-fit by a single power-law, with a well-constrained index that is retrieved from a fit of the GBM data alone, the LAT data alone, and when jointly fitting the entire data set. The parameters for this multi-component fit are Band_alpha = 0.61 ± 0.01, Band_beta = 3.87 ± 0.16, Band_EPeak = 798 ± 7 keV, and a power-law index of 1.94 ± 0.01 that shows no evidence for a spectral cut-off below 100 GeV. The fluence between 8 keV and 30 GeV is 4.86 ± 0.06 x 10-4 ergs/cm2. The points of contact for this burst are: Francesco de Palma (francesco.depalma@ba.infn.it) and Elisabetta Bissaldi (ebs@mpe.mpg.de). 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. This message can be cited.