Resistive relaxation of a magnetically confined mountain on an accreting neutron star

Abstract

Three-dimensional numerical magnetohydrodynamic (MHD) simulations are performed to investigate how a magnetically confined mountain on an accreting neutron star relaxes resistively. No evidence is found for non-ideal MHD instabilities on a short time-scale, such as the resistive ballooning mode or the tearing mode. Instead, the mountain relaxes gradually as matter is transported across magnetic surfaces on the diffusion time-scale, which evaluates to ${\tau }_{\mathrm{I}}\sim 10^5-10^8$ yr (depending on the conductivity of the neutron star crust) for an accreted mass of $M_a=1.2\times 10^{-4}{M}_{\odot }$. The magnetic dipole moment simultaneously reemerges as the screening currents dissipate over ${\tau }_{\mathrm{I}}$. For nonaxisymmetric mountains, ohmic dissipation tends to restore axisymmetry by magnetic reconnection at a filamentary neutral sheet in the equatorial plane. Ideal-MHD oscillations on the Alfvén time-scale, which can be excited by external influences, such as variations in the accretion torque, compress the magnetic field and hence decrease ${\tau }_{\mathrm{I}}$ by \change{one order of magnitude} relative to its standard value (as computed for the static configuration). The implications of long-lived mountains for gravitational wave emission from low-mass X-ray binaries are briefly explored.

arXiv commentaccepted by MNRAS

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