Accretion by magnetic neutron stars. I - Magnetospheric structure and stability

Abstract

This paper is the first of a series in which we present the results of a study of accretion by a slowly rotating, magnetic neutron star when the accretion flow is approximately radial. In this study we examine in detail the physical processes that occur in the neighborhood of the magneto- spheric boundary and the manner in which accreting plasma enters the magnetosphere. Here we investigate the conditions necessary for the formation of a magnetospheric cavity and for its stability. We find that although the boundary of the cavity is initially close to instability, magneto- spheric models strongly suggest that it is stable until the plasma outside undergoes at least some cooling. Assuming that little plasma enters the magnetosphere when the boundary is stable, we show that there is, typically, time for the plasma outside to form a quasi-static atmosphere around the magnetosphere before instability sets in. We discuss the scale and structure of such static configurations, including the nature of the current layer at the magnetospheric boundary. We also investigate the structure of the magnetosphere when there is a steady inflow of plasma across the boundary, and describe the possible flow patterns at the boundary and in the interior. Assuming that plasma enters the magnetosphere predominantly via Rayleigh-Taylor instability of the boundary, we discuss the conditions necessary for a steady accretion flow and note that beaming of X-rays from the stellar surface can strongly affect the plasma flow pattern near the boundary. We show that the Alfvén surface, within which the stellar field channels the flow, generally does not coincide with the magnetospheric boundary and could conceivably lie well within it. A discussion of the applicability of the present study to observed compact X-ray sources shows that it may apply directly to the longer-period pulsating sources, such asAlllS-61,3U 0900-40, A0535 + 26, and 3U 1728-24, if the accretion flow is sufficiently radial, and to some X-ray burst sources, if these are slowly rotating neutron stars.