GAMMA-RAY SIGNAL FROM THE PULSAR WIND IN THE BINARY PULSAR SYSTEM PSR B1259–63/LS 2883

Abstract

Binary pulsar systems emit potentially detectable components of gamma ray emission due to Comptonization of the optical radiation of the companion star by relativistic electrons of the pulsar wind, both before and after termination of the wind. The recent optical observations of binary pulsar system PSR B1259-63/LS 2883 revealed radiation properties of the companion star which differ significantly from previous measurements. In this paper we study the implications of these observations for the interaction rate of the unshocked pulsar wind with the stellar photons and the related consequences for fluxes of high energy (HE) and very high energy (VHE) gamma rays. We show that the signal should be strong enough to be detected with Fermi close to the periastron passage, unless the pulsar wind is strongly anisotropic or the Lorentz factor of the wind is smaller than $10^3$ or larger that $10^5$. The higher luminosity of the optical star also has two important implications: (i) attenuation of gamma rays due to photon-photon pair production, and (ii) Compton drag of the unshocked wind. While the first effect has an impact on the lightcurve of VHE gamma rays, the second effect may significantly decrease the energy available for particle acceleration after termination of the wind.