Post-periastron ASCA observation of the PSR B1259-63 system

We report on the results of a post-periastron ASCA observation of the PSR B1259-63 system, containing a 48 ms pulsar orbiting a Be star in an eccentric 3.4 yr orbit. The ASCA observation was carried out on 1994 February 28 when the radio pulsar was again visible after a prolonged 40-day radio eclipse near to periastron. The source was clearly detected with an X-ray luminosity (1-10 keV) L(X) of (1.03 +/- 0.09) x 10(34) (d/2 kpc)(2) erg s(-1), where d is the pulsar's distance. The X-ray spectrum is well-represented by a single power-law model of the photon index, alpha = 1.61 +/- 0.06, and a small photoelectric absorption, N-H = (5.6 +/- 0.6) x 10(21) cm(-2). No significant X-ray pulsations were detected, and the upper limit for the pulsed component was estimated to be 15% of the total X-ray flux: assuming sinusoidal modulation. The characteristics of the X-ray emission detected on 1994 February 28 are similar to those detected by ASCA near periastron when the pulsed radio emission from PSR B1259-63 was eclipsed. Our results strongly favor a non-thermal model of X-ray emission driven by synchrotron radiation from relativistic shocked particles in the pulsar wind interacting with the outflow from the Be star companion.