Observing the unique binary PSR B 1259 − 63 / LS 2883

A new class of high-mass X-ray binaries (HMXBs) have been recently discovered as γ-ray emitters. These binaries contain a compact object orbiting an OB companion star. It is commonly believed that energetic particles are either accelerated in jets or in the collision shocks between the stellar wind and pulsar wind, in turn upscattering the optical to UV lights to GeV–TeV γ-rays. Pulsed γ-rays from pulsar magnetosphere might also contribute to the observed photons up to a few GeV. Several groups of authors have proposed phenomenological models to explain the high-energy emission behaviors of γ-ray binaries (e.g., Dubus 2006, Takata & Taam 2009, Kong et al. 2011, Zabalza et al. 2013). Up to now, the following five systems are confirmed GeV and/or TeV γ-ray binary systems (in ascending order of the orbital periods): LS 5039 (3.9 days), 1FGL J1018.6−5856 (16.6 days), LS I +61 303 (26.5 days), HESS J0632+057 (321 days), and PSR B1259−63 (3.4 years). For LS 5039 and LS I +61 303, GeV and/or TeV γ-ray emission are modulated on the orbital period but is easily detected throughout the whole orbit. In the case of PSR B1259-63, γ-ray emission is only detected in certain parts of the orbit. Together with its long orbital period (3.4 years), it was only detected in one occasion when the pulsar approached the companion star in late 2010/early 2011. Being a transient source, it is important to capture those γ-ray signal close to its periastron once every 3.4 years. Therefore, we propose to observe PSR B1259−63, a γ-ray binary with a 3.4-year orbital periods, so as to maximize the scientific return of the Fermi/LAT instrument during its mission life.