X-ray/gamma-ray observations of the PSR B1259-63/SS 2883 system near apastron

We report the results from X-ray and hard X-ray observations of the PSR B12 59-63/SS 2883 system with ASCA and the Compton Gamma-Ray Observatory (CGRO) , performed between 1995 February and 1996 January when the pulsar was near apastron. The system was clearly detected in each of the two ASCA observat ions with luminosity in the 1-10 keV band of L-X = (9 +/- 3) x 10(32) (d/2 kpc)(2) ergs s(-1) while CGRO/OSSE did not detect significant hard X-rays f rom the system. X-ray spectra obtained with ASCA are well fitted by a singl e power-law spectrum with a photon index of 1.6 +/- 0.3. No pulsations were detected in either the ASCA or the OSSE data. We combine all existing X-ra y and hard X-ray observations and present the orbital modulation in the lum inosity and photon index for the entire orbit. The results are in agreement with predictions based on a synchrotron emission model from relativistic p articles in a shocked pulsar wind interacting with the gaseous outflow from the Be star.