Plasma acceleration by radiation in X-ray pulsars

Abstract

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 velocityv≈0.65c. These results are applied to plasma acceleration inX-ray pulsars. The mechanism is advanced, of γ-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 (γ≈2.6) electron-positron pairs and are transformed into γ-radiation. Hard quanta with energy ℰ∼1 MeV leave the generation region in the narrow cone ϑ≲0.25.