The binary system consisting of a radio pulsar PSR B1259-63 and a Be star SS 2883 has been observed with the ASCA satellite at six epochs, from the most recent periastron passage in January 1994 to the recent apastron approach in 1995. In this unique system, shock acceleration of the relativistic pulsar wind is expected to result in high-energy emission. Since the distance between the pulsar and the Be star varies by over one order of magnitude during its orbital period of 3.4 year, the high-energy emission from the system provides a unique chance to study the emission mechanism under predictably varying conditions imposed by the Keplerian orbit. In this thesis for the first time we challenge this exciting possibility and sort out contributions of properties of stellar winds.
The system is unambiguously detected in the X-ray band ranging from 1 keV to 10 keV at all six epochs, with moderate X-ray luminosity Lx = (0.1-1)E+34 erg sE-1 and little absorption. We detect no X-ray pulsations at any of six observed epochs. The observed X-ray spectra are generally consistent with a single power law model but the luminosities in the band between 1 and 10 keV differ in all six observations. At periastron the spectrum shows some softening and the intensity drops to about a half of those observed two weeks before and after the periastron. The column density determined from absorption is NH = 6E+21 cmE-2 for all six observations, consistent with that expected from the galactic contribution alone.
Based on the observations we examine contributions of possible mechanisms for X-ray emission in the system, namely, coronal emission from the Be star, rotation-powered emission from the pulsar, emission from accretion onto the neutron star surface, emission from accreted materials intercepted by the pulsar's magnetosphere, and emission from a shock front formed at the location where the ram pressures of the Be stellar wind and the pulsar wind balance. Among them, synchrotron X-rays emitted from relativistic electron/positron pairs accelerated non-thermally in the shock region accounts naturally for the observations. Within the framework of this model, we show that inverse Compton cooling by UV photons from the Be star is important at periastron, and the shock acceleration must create e+- pairs with energy index s = 2 up to the Lorentz factor of 1E+7 within about 100 s or a shorter time just behind the shock front. In addition, we find evidence for a large inclination of the pulsar's orbit with respect to the equatorial plane of the Be star. From energetics in the shock-powered emission model, we find that the pulsar wind carries more than 4E+35 erg sE-1, a half of the spin-down luminosity of the pulsar. We also find, from the amount of the decrease in X-ray flux at the apastron approach, the momentum flux of the stellar wind lies between 0.05 and 0.28 in units of (1E-8 Msolar yrE-1})(1000 km sE-1) at apastron.