Fermi-Large Area Telescope Results on Pulsar Wind Nebulae after 1.5 years of Observations

Prior to the Fermi Gamma-ray Space Telescope, only six pulsars and one associated pulsar wind nebula, the Crab Nebula, had been detected in gamma-rays by EGRET. Since then, the Large Area Telescope (LAT) aboard Fermi has significantly increased the number of detected pulsars in the 100 MeV to 30 GeV energy range. The new pulsar population established by early LAT observations show that we are detecting many nearby young pulsars in addition to the population of millisecond pulsars. All Fermi-LAT pulsars have high energy loss rate. A large fraction of these pulsars are associated to pulsar wind nebulae candidates observed in the TeV energy range by Cherenkov telescopes. These pulsars are thus likely to power a PWN detectable by Fermi, as it has already been done for the Crab. We will review the results obtained so far on pulsar wind nebulae, including the very famous Crab Nebula, Vela-X and MSH 15-52, and give a general overview of the constraints provided by Fermi-LAT non-detection after 1.5 year of observations.