  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.
