A model of nonthermal radiation from accreting X-ray pulsars

Abstract

The formation of accelerator gaps in the magnetosphere of accreting X-ray pulsars, especially in the regions above the inner part of the accreting disk has been proposed. In this paper, we concentrate on the case where the gaps are filled with soft X-ray photons which arise from the accretion-powered thermal X-rays at the polar caps, but are Compton-degraded when they traverse the accretion matter flow. The high-energy electron current, whose existence is assumed to counter balance the positive ion (proton) flow, boosts these soft X-rays up to γ-rays by inverse Compton scattering. Only very few of these γ-rays, however, can be directly observed on Earth because most of them collide with the thermal X-ray photons from the polar caps outside the gaps. This generates e± pairs which, in turn, will emit a new generation of nonthermal X-rays by synchrotron radiation. In some sources this nonthermal X-ray spectrum should be detectable. Examples are worked out for Her X-1, 4U 0115+63, Vel X-1, Cen X-3 and Cyg X-3, and they show that the modeled spectra may be observed at photon energies either smaller than a few tenths of keV and/or larger than a few tens of keV.