High-Energy Emission from the PSR B1259-63 System near Periastron

We report the results of a CGRO 3-week observation of the binary system containing the 47 ms pulsar PSR B1259-63 orbiting around a Be star companion in a very eccentric orbit. The PSR B1259-63 binary is a unique system for the study of the interaction of a rapidly rotating pulsar with time-variable nebular surroundings. CGRO observed the PSR B1259-63 system in coincidence with its most recent periastron passage (January 3-23, 1994). Unpulsed and non-thermal hard X-ray emission was detected up to 200 keV, with a photon index $1.8 \pm 0.2$ and a flux of ~4 mCrab, corresponding to a luminosity of a few 10^{34} erg/s at the distance of 2 kpc. The hard X-ray flux and spectrum detected by CGRO agrees with the X-ray emission measured by simultaneous ASCA observations. EGRET upper limits are significant, and exclude strong inverse Compton cooling in the PSR B1259-63 system. We interpret the observed non-thermal emission as synchrotron radiation of shocked electron/positron pairs of the relativistic pulsar wind interacting with the mass outflow from the Be star. Our results clearly indicate, for the first time in a binary pulsar, that high energy emission can be shock-powered rather than caused by accretion. The lack of X-ray/gamma-ray pulsations constrains models of high-energy emission from rapidly rotating pulsars.