The Limiting Luminosity of Accreting Neutron Stars With Magnetic Fields

Abstract

Accretion on to a magnetized neutron star for high accretion rates, when one can no longer ignore the back-reaction of emergent light on the infalling material, is discussed in detail. The equations of hydrodynamics and radiative diffusion are solved in a one-dimensional approximation; the solution is expressed in an analytical form. The luminosity L⋆ is evaluated beyond which one should allow for the dynamic effect of emergent light on the infalling material. The limiting X-ray luminosity L⋆⋆ of accreting magnetized neutron stars is shown to depend crucially on the geometry of the accretion channel. Under certain conditions the value of L⋆⋆ may appreciably exceed the critical Eddington value LEd, the main energy flux being carried away by neutrinos. The effects connected with the gas flow along the magnetospheric surface are discussed in detail. The plasma layer on the Alfvén surface is shown to be optically thick with respect to Thomson scattering and to reradiate in soft X-rays |$h_{v} \lesssim 1$| ke V a considerable fraction of the primary X-ray flux. A necessary condition for the X-ray luminosity to exceed the Eddington limit is a certain degree of asymmetry in the distribution of matter over the Alfvén surface. In the framework of the adopted model, regular pulsations of hard and soft X-rays are discussed.