Plasma Physics of Accreting Neutron Stars

Abstract

Accretion of plasma by a neutron star is thought to be the fundamental process that powers many X- and γ-ray sources in the cosmos. Examples of such sources include the X-ray pulsars found in high- and low-mass binary systems and the luminous X-ray stars found in low-mass binary systems in the galactic bulge. These lectures provide an introduction to some of the plasma concepts and phenomena that are needed to understand such sources. The capture of material from the wind or from the atmosphere or envelope of a binary companion star is described and the resulting types of accretion flows discussed. The reasons for the formation of a magnetosphere around the neutron star are explained. The qualitative features of the magnetospheres of accreting neutron stars are then described and compared with the qualitative features of the geomagnetosphere. The conditions for stable flow and for angular and linear momentum conservation are explained in the context of accretion by magnetic neutron stars and applied to obtain rough estimates of the scale of the magnetosphere. Accretion from Keplerian disks is then considered in some detail. The radial structure of geometricallythin disk flows, the interaction of disk flows with the neutron star magnetosphere, and models of steady accretion from Keplerian disks are described. Accretion torques and the resulting changes in the spin frequencies of rotating neutron stars are considered. The predicted behavior is then compared with observations of accretion-powered pulsars. Magnetospheric processes that may accelerate particles to very high energies, producing GeV and, perhaps, TeV γ-rays are discussed.Finally, the mechanisms that decelerate and eventually stop accreting plasma at the surfaces of strongly magnetic neutron stars are described.