Anisotropic inverse Compton scattering from the circumstellar disc in PSR B1259−63

The gamma-ray binary system PSR B1259−63 has been detected from radio to very high energy gamma-rays. The presence of an extended circumstellar disc around the Be optical companion suggests the possibility that the IR flux from the disc will provide additional target photons for inverse Compton gamma-ray production and could result in an increase in the gamma-ray flux, particularly at GeV energies. The extent of the IR excess is modelled by making use of a curve of growth method, which is fitted to optical and near-IR archive observations and mid-IR observations obtained with the VLT on 5 January 2011. The anisotropic inverse Compton emission is modelled around periastron by assuming a constant, adiabatically cooled electron distribution. The results show that the IR flux can increase the GeV emission by a factor > 2, with the maximum contribution occurring at periastron and not near the disc crossing epoch. A flare from PSR B1259−63 was detected with the Fermi telescope with an implied luminosity of ≈ 100 per cent of the spin-down luminosity of the pulsar, approximately 30 days after the 2010 periastron. This unexpected flare cannot be explained by this model without incorporating additional effects to greatly increase the IR photon density near the disc crossing.