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Abstract
We report on gamma-ray observations in the off-pulse window of the Vela pulsar PSR
B0833-45, using 11 months of survey data from the Fermi Large Area Telescope (LAT).
This pulsar is located in the 8 degree diameter Vela supernova remnant, which contains
several regions of non-thermal emission detected in the radio, X-ray and gamma-ray
bands. The gamma-ray emission detected by the LAT lies within one of these regions,
the 2*3 degrees area south of the pulsar known as Vela-X. The LAT flux is signicantly
spatially extended with a best-fit radius of 0.88 +/- 0.12 degrees for an assumed
radially symmetric uniform disk. The 200 MeV to 20 GeV LAT spectrum of this source
is well described by a power-law with a spectral index of 2.41 +/- 0.09 +/- 0.15 and
integral flux above 100 MeV of (4.73 +/- 0.63 +/- 1.32) * 10^{-7} cm^{-2} s^{-1}.
The first errors represent the statistical error on the fit parameters, while the
second ones are the systematic uncertainties. Detailed morphological and spectral
analyses give strong constraints on the energetics and magnetic field of the pulsar
wind nebula (PWN) system and favor a scenario with two distinct electron populations.