Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2009-02-25
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
accepted by MNRAS
Scientific paper
10.1111/j.1365-2966.2009.14698.x
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 Alfv\'{e}n 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.
Melatos Andrew
Vigelius Matthias
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