  The binary system PSR B1259-63/LS 2883 consists of a 47.8 ms radio pulsar
that orbits the companion Be star with a period of 3.4 years in a highly
eccentric orbit. The system is well sampled in radio, X-rays, and TeV
gamma-rays, and shows orbital-phase-dependent variability in all observed
frequencies. Here we report on the discovery of >100 MeV gamma-rays from PSR
B1259-63/LS 2883 through the 2010 pariastron passage. Using data collected with
the Large Area Telescope aboard Fermi from 33 days before pariastron to 75 days
after pariastron, PSR B1259-63/LS 2883 is detected at a significance of 13.6
standard deviations. The gamma-ray light curve is highly variable over the
above period, with changing photon index that correlates with gamma-ray flux.
In particular, two major flares that occur after the pariastron passage were
observed. The onset of gamma-ray emission occurs close to, but not at the same
orbital phases as, the two disk passages that occur ~1 month before and ~1
month after the pariastron passage. The fact that the GeV orbital light curve
is different from that of the X-ray and TeV light curves strongly suggests that
GeV gamma-ray emission originates from a different component. We speculate that
the observed GeV flares may be resulting from Doppler boosting effects.
