X-Ray Detection of PSR B1259-63 at Periastron

We report on results of X-ray observations of the unique radio pulsar/Be star binary PSR B1259 - 63, obtained using the ASCA satellite 2 weeks before, during, and 2 weeks after the pulsar's most recent periastron passage on 1994 January 9. The source was detected at all three epochs, with an X-ray luminosity in the 1-10 keV band of 1 × 1034(d/2 kpc)2 ergs s-1 at the pre- and postperiastron epochs, and a factor of ∼2 smaller at periastron. The X-ray emission can be characterized by power-law spectra with a photon index in the range 1.5-1.9, with evidence for spectral softening at periastron. The photoelectric absorption, NH ∼ 5 × 1021 cm-2, was constant for the three observations within measurement uncertainties and is consistent with the Galactic contribution. We detect no pulsations and derive an upper limit on periodicities close to the PSR B1259 - 63 spin period of ∼7% of the total observed flux, conservatively assuming a sine wave profile. We argue that accretion of gaseous material onto the surface of the neutron star is an unlikely origin for the observed X-rays. The characteristics of the X-ray emission from the PSR B1259 -63 system are in good qualitative agreement with models of nonthermal acceleration of relativistic particles from the pulsar wind in a shock at the location at which the pulsar and Be star wind pressures balance. Assuming reasonable Be star equatorial outflow and pulsar wind pressures, we find that the mass-loss rate Mṡ and surface velocity υ are constrained by the relation 100 ≲ (υ/10 km s-1)(Mṡ/10-8 Msun yr-1) ≲ 104.