Theory of High‐Energy Emission from the Pulsar/Be Star System PSR 1259−63. I. Radiation Mechanisms and Interaction Geometry

We study the physical processes of the PSR B1259-63 system containing a 47 mspulsar orbiting around a Be star in a highly eccentric orbit. Motivated by theresults of a multiwavelength campaign during the January 1994 periastronpassage of PSR B1259-63, we discuss several issues regarding the mechanism ofhigh-energy emission. Unpulsed power law emission from the this system wasdetected near periastron in the energy range 1-200 keV. We find that theobserved high energy emission from the PSR B1259-63 system is not compatiblewith accretion or propeller-powered emission. Shock-powered high-energyemission produced by the pulsar/outflow interaction is consistent with all highenergy observations. By studying the evolution of the pulsar cavity weconstrain the magnitude and geometry of the mass outflow outflow of the Bestar. The pulsar/outflow interaction is most likely mediated by a collisionlessshock at the internal boundary of the pulsar cavity. The system shows all thecharacteristics of a {\it binary plerion} being {\it diffuse} and {\it compact}near apastron and periastron, respectively. The PSR B1259-63 cavity is subjectto different radiative regimes depending on whether synchrotron or inverseCompton (IC) cooling dominates the radiation of electron/positron pairsadvected away from the inner boundary of the pulsar cavity. The highlynon-thermal nature of the observed X-ray/gamma-ray emission near periastronestablishes the existence of an efficient particle acceleration mechanismwithin a timescale shown to be less than $\sim 10^2-10^3$ s. A synchrotron/ICmodel of emission of e\pm-pairs accelerated at the inner shock front of thepulsar cavity and adiabatically expanding in the MHD flow provides an excellentexplanation of the observed time variableX-ray flux and spectrum from the PSRComment: 68 pages, accepted for publication in the Astrophys. J. on Aug. 26, 199