The Vela Pulsar: Results from the First Year of Fermi LAT Observations
  We report on analysis of timing and spectroscopy of the Vela pulsar using
eleven months of observations with the Large Area Telescope on the Fermi
Gamma-Ray Space Telescope. The intrinsic brightness of Vela at GeV energies
combined with the angular resolution and sensitivity of the LAT allow us to
make the most detailed study to date of the energy-dependent light curves and
phase-resolved spectra, using a LAT-derived timing model. The light curve
consists of two peaks (P1 and P2) connected by bridge emission containing a
third peak (P3). We have confirmed the strong decrease of the P1/P2 ratio with
increasing energy seen with EGRET and previous Fermi LAT data, and observe that
P1 disappears above 20 GeV. The increase with energy of the mean phase of the
P3 component can be followed with much greater detail, showing that P3 and P2
are present up to the highest energies of pulsation. We find significant pulsed
emission at phases outside the main profile, indicating that magnetospheric
emission exists over 80% of the pulsar period. With increased high-energy
counts the phase-averaged spectrum is seen to depart from a power- law with
simple exponential cutoff, and is better fit with a more gradual cutoff. The
spectra in fixed-count phase bins are well fit with power-laws with exponential
cutoffs, revealing a strong and complex phase dependence of the cutoff energy,
especially in the peaks. By combining these results with predictions of the
outer magnetosphere models that map emission characteristics to phase, it will
be possible to probe the particle acceleration and the structure of the pulsar
magnetosphere with unprecedented detail.
