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

We investigate the atmospheres, oceans, and crusts 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 M_☉ yr^-1 (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 10⁸ yr) from the values characteristic of the colder crusts in slowly accreting neutron stars. As suggested by Konar & Bhattacharya, at high accretion rates the flow timescale competes with the Ohmic diffusion time in determining the evolution of the crust magnetic field. At a global accretion rate of ≈ 3 × 10⁻⁹ M_☉ yr^-1, the Ohmic diffusion time across a scale height equals the flow time over a large range of densities in the outer crust. In the inner crust (below the neutron drip), the diffusion time is always longer than the flow time for sub-Eddington accretion rates. 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 10 times the Eddington rate. This rapid accretion heats the atmosphere/ocean to temperatures of order 10⁹ 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 recur 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. We estimate how large such a confined "mountain" could be and show that the currents needed to confine the mountain are large enough to modify, by order unity, the magnetic field strength at the polar cap. If the mountain's structure varies in time, the changing surface field could cause temporal changes in the pulse profiles and cyclotron line energies of accreting X-ray pulsars.