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

Abstract

The gamma-ray binary system PSR B1259−63/LS 2883 consists of a 48 ms pulsar orbiting a Be star. The system is particularly interesting because it is the only gamma-ray binary system where the nature of the compact object is known. The non-thermal radiation from the system is powered by the spin-down luminosity of the pulsar and the unpulsed radiation originates from the stand-off shock front which forms between the pulsar and stellar wind. The Be star/optical companion in the system produces an excess infrared (IR) flux from the associated circumstellar disc. This IR excess provides an additional photon source for inverse Compton scattering. We discuss the effects of the IR excess near periastron, for anisotropic inverse Compton scattering and associated gamma-ray production. We determine the IR excess from the circumstellar disc using a modified version of a curve of growth method, which takes into account the changing optical depth through the circumstellar disc during the orbit. The model is constrained using archive data and additional mid-IR observations obtained with the Very Large Telescope (VLT) during 2011 January. The inverse Compton scattering rate was calculated for three orientations of the circumstellar disc. The predicted gamma-ray light curves show that the disc contribution is a maximum around periastron and not around the disc crossing epoch. This is a result of the disc being brightest near the stellar surface. Additional spectroscopic and near-IR observations were obtained of the system and these are discussed in relation to the possibility of shock heating during the disc crossing epoch.

Multifrequency Behaviour of the Gamma-Ray Binary System PSR B1259-63: Modelling the FERMI Flare

“…The simulations show that the disc should be considerably disrupted during the pulsar crossing. However, observations on 5 January 2011, approximately three weeks after periastron, showed that the mid-infrared flux was consistent with previous observations (see van Soelen et al, 2012). This suggests that either the disc was not dramatically effected, or that there was a rapid recovery.…”

Gamma-rays from the binary system containing PSR J2032+4127 during its periastron passage

“…In the case of the binary system PSR B1259-63 the decretion disk extends up to the distance of the pulsar periastron. In such a case, the gamma-ray emission above ∼ TeV energy range, can be also produced in the comptonization of the disk radiation [61,62]. If this occurs also for the considered binary system, then the steady, extended TeV emission from the HEGRA TeV γ-ray source (TeV J2032+4130) can be overcome by the point-like emission from the binary system also above a few TeV during the periastron passage of the pulsar.…”

Pair cascades in the disk environment of the binary system PSR B1259-63/LS 2883

“…at the corresponding distance of 45R * (the separation distance at ∼ 20 days after periastron (see e.g. van Soelen et al, 2012) is, however, comparable to the one at periastron. Therefore, in the disk η is (10 2 − 10 4 ) lower than at periastron resulting in the distance to the termination shock of ρ ∼ (0.001 − 0.01)d = (0.05 −0.5)R * .…”