Plasma acceleration by radiation in X-ray pulsars

Charged particle acceleration is considered by a radiation flux from a star or hot spot in X-ray pulsars. It is shown that for any distance from the star there exists the upper velocity limit up to which a particle can be accelerated by radiation. This critical velocity does not depend on the luminosity of the spot. Near the hot spot surface the critical velocityv0.65c. These results are applied to plasma acceleration inX-ray pulsars. The mechanism is advanced, of -ray generation in the course of plasma accretion, onto a neutron star. It is shown that in the presence of a large magnetic field and high luminosity of the spot the relativistic electron-position avalanche may appear. The optical depth of the electron-positron cloud achieves the value of order one. The X-ray quanta emitted by the spot are scattered by relativistic (2.6) electron-positron pairs and are transformed into -radiation. Hard quanta with energy 1 MeV leave the generation region in the narrow cone 0.25.