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GCN Circular 9872

Subject
GRB 090902B: Fermi LAT and GBM refined analysis.
Date
2009-09-03T07:36:42Z (15 years ago)
From
Valerie Connaughton at MSFC <valerie@nasa.gov>
F. de Palma (Universit� 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.

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