PROBING THE PULSAR WIND IN THE γ-RAY BINARY SYSTEM PSR B1259–63/SS 2883

Abstract

The spectral energy distribution from the X-ray to the very high energy regime (>100 GeV) has been investigated for the γ-ray binary system PSR B1259–63/SS 2883 as a function of the orbital phase within the framework of a simple model of a pulsar wind nebula. The emission model is based on the synchrotron radiation process for the X-ray regime and the inverse Compton scattering process boosting stellar photons from the Be star companion to the very high energy (100 GeV-TeV) regime. With this model, the observed temporal behavior can, in principle, be used to probe the pulsar wind properties at the shock as a function of the orbital phase. Due to theoretical uncertainties in the detailed microphysics of the acceleration process and the conversion of magnetic energy into particle kinetic energy, the observed X-ray data for the entire orbit are fitted using two different methods. In the first method, the magnetization parameter and the Lorentz factor of the wind at the shock are allowed to vary for a given power law index characterizing the accelerated particles at the shock. In this case, the observed photon index of ~1.2 in the 1-10 keV energy band near the periastron passage can be understood provided that (1) the electron energy distribution is described by a broken power law and (2) there is a break at an energy of about 8 × 106 in units of the electron rest mass energy. In the second method, the magnetization parameter and the power law index are varied for a fixed Lorentz factor. Here, the photon index of ~1.2 can result from a particle distribution described by a power law index of ~1.5. The calculated emission in the energy band corresponding to 10 MeV-1 GeV from the shocked pulsar wind indicates that these two cases can be distinguished by future Fermi observations near the periastron. It is also found that the emission from the unshocked wind could be detectable by the Fermi telescope near the periastron passage if most of the kinetic energy of the flow is carried by particles with Lorentz factors of Γ ~ 105.