Detections of millisecond pulsars with the Fermi Large Area Telescope
  The Fermi observatory was launched on June 11, 2008. It hosts the \emph{Large
Area Telescope} (LAT), sensitive to $\gamma$-ray photons from 20 MeV to over
300 GeV. When the LAT began its activity, nine young and energetic pulsars were
known in $\gamma$ rays. At least several tens of pulsar detections by the LAT
were predicted before launch. The LAT also allowed the study of millisecond
pulsars (MSPs), never firmly detected in $\gamma$ rays before Fermi.
  This thesis first presents the pulsar timing campaign for the LAT, in
collaboration with large radiotelescopes and X-ray telescopes, allowing for
high sensitivity pulsed searches. Furthermore, it lead to quasi-homogeneous
coverage of the galactic MSPs, so that the search for pulsations in LAT data
for this population of stars was not affected by an \emph{a priori} bias.
  We present a search for pulsations from these objects in LAT data. For the
first time, eight galactic MSPs have been detected as sources of pulsed
$\gamma$-ray emission over 100 MeV. In addition, a couple of good candidates
for future detection are seen. A similar search for globular cluster MSPs has
not succeeded so far. Comparison of the phase-aligned $\gamma$-ray and radio
light curves, as well as the spectral shapes, leads to the conclusion that
their $\gamma$-ray emission is similar to that of normal pulsars, and is
probably produced in the outer-magnetosphere. This discovery suggests that many
unresolved $\gamma$-ray sources are unknown MSPs.
