The Ocean and Crust of a Rapidly Accreting Neutron Star: Implications for Magnetic Field Evolution and Thermonuclear Flashes

Abstract

We investigate the atmosphere, ocean, and crust of neutron stars accreting at rates sufficiently high (typically in excess of the local Eddington limit) to stabilize the burning of accreted hydrogen and helium. For hydrogen-rich accretion at global rates in excess of 10^-8 solar masses per year (typical of a few neutron stars), we discuss the thermal state of the deep ocean and crust and their coupling to the neutron star core, which is heated by conduction (from the crust) and cooled by neutrino emission. We estimate the Ohmic diffusion time in the hot, deep crust and find that it is noticeably shortened (to less than 10^8 yr) from the values characteristic of the colder crusts in slowly accreting neutron stars. We speculate on the implications of these calculations for magnetic field evolution in the bright accreting X-ray sources. We also explore the consequences of rapid compression at local accretion rates exceeding ten times the Eddington rate. This rapid accretion heats the atmosphere/ocean to temperatures of order 10^9 K at relatively low densities; for stars accreting pure helium, this causes unstable ignition of the ashes (mostly carbon) resulting from stable helium burning. This unstable burning can re-occur on timescales as short as hours to days, and might be the cause of some flares on helium accreting pulsars, in particular 4U~1626--67. Such rapid local accretion rates are common on accreting X-ray pulsars, where the magnetic field focuses the accretion flow onto a small fraction of the stellar area.