  Three periastron passages of the PSR B1259$-$63/LS 2883 binary system,
consisting of a 48 ms rotation-powered pulsar and a $\sim30$ M$_{\odot}$ Be
star, have been observed by the Large Area Telescope (LAT) on board the Fermi
Gamma-ray Space Telescope, in 2010, 2014, and 2017. During the most-recent
periastron passage, sustained low-level gamma-ray emission was observed over a
$\sim3$-week long interval immediately after periastron, which was followed by
an interval of no emission. Sporadic flares were detected starting 40 days
post-periastron and lasted approximately 50 days, during which the emission
displayed significant spectral curvature, variability on timescales as short as
1.5 minutes, and peak flux levels well in excess of the pulsar spin-down power.
By contrast, during the 2010 and 2014 periastron passages, significant
gamma-ray emission was not observed with the LAT until 30 and 32 days
post-periastron, respectively. The previous flares did not exhibit spectral
curvature, showed no short term variability, and did not exceed the pulsar
spin-down power. The high flux and short timescales observed in 2017 suggest
significant beaming of the emission is required and constrain the size of the
emission region. The flares occur long enough after periastron that the neutron
star should already have passed through the extended disk-like outflow, thus
constraining options for target material and seed photon sources for inverse
Compton models.
