HIGH-ENERGY EMISSION FROM THE PSR B1259-63 SYSTEM NEAR PERIASTRON

We report the results of a CGRO multi-instrument 3-week observation of the binary system containing the 47 ms pulsar PSR B1259-63 orbiting a round 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 rapidl y rotating pulsar with time-variable nebular surroundings. CGRO observ ed the PSR B1259-63 system in coincidence with its most recent periast ron passage (January 3-23, 1994). Unpulsed and nonthermal hard X-ray e mission was detected up to 200 keV, with a photon index 1.8+/-0.2 and a flux of similar to 4 mCrab, corresponding to a luminosity of a few x 10(34) erg/s at the distance of 2 kpc. The hard X-ray flux and spectr um detected by CGRO agrees with the X-ray emission measured by simulta neous ASCA observations. EGRET upper limits are significant, and exclu de strong inverse Compton cooling in the PSR B1259-63 system. We inter pret the observed non-thermal emission as synchrotron radiation of sho cked electron/positron pairs of the relativistic pulsar wind interacti ng with the mass outflow from the Be star. Our results clearly indicat e, for the first time in a binary pulsar, that high energy emission ca n be shock-powered rather than caused by accretion. Furthermore, the P SR B1259-63 system shows that shock acceleration can increase the orig inal energy of pulsar wind particles by a factor greater than or simil ar to 10, despite the high synchrotron and inverse Compton cooling rat es near periastron. We also present results of an extensive search for pulsed gamma-ray emission from PSR B1259-63. The lack of pulsations c onstrains models of gamma-ray emission from rapidly rotating pulsars.