XMM-Newton observations of PSR B1259-63 near the 2004 periastron passage

PSR B1259-63 is in a highly eccentric 3.4-yr orbit with a Be star and crosses the Be star disc twice per orbit, just prior to and just after periastron. Unpulsed radio, X-ray and gamma-ray emission observed from the binary system is thought to be due to the collision of pulsar wind with the wind of Be star. We present here the results of new XMM-Newton observations of the PSR B1259-63 system during the beginning of 2004 as the pulsar approached the disc of the Be star. We combine these results with the earlier unpublished X-ray data from BeppoSAX and XMM-Newton as well as with the ASCA data. The detailed X-ray light curve of the system shows that the pulsar passes (twice per orbit) through a well-defined Gaussian-profile disc with the half-opening angle (projected on the pulsar orbit plane) . The intersection of the disc middle plane with the pulsar orbital plane is inclined at θ_disc~= 70° to the major axis of the pulsar orbit. Comparing the X-ray light curve to the TeV light curve of the the system, we find that the increase of the TeV flux some 10-100d after the periastron passage is unambiguously related to the disc passage. At the moment of entrance to the disc, the X-ray photon index hardens from Γ~= 1.8 up to ~=1.2 before returning to the steeper value Γ>= 1.5. Such behaviour is not easily accounted for by the model in which the X-ray emission is synchrotron emission from the shocked pulsar wind. We argue that the observed hardening of the X-ray spectrum is due to the inverse-Compton or bremsstrahlung emission from 10-100 MeV electrons responsible for the radio synchrotron emission.