Residual ellipticity driven by intense magnetic stress in the hot neutron star era

A neutron star with a nonaxisymmetric mass distribution continuously radiates gravitational waves. The amplitude depends on the magnitude of the deformation, and is crucial for gravitational-wave astronomy. We studied an evolutionary effect on the magnetically induced ellipticity in the neutron star crust. A large deformation at the hot era before solidification is assumed to be produced magneto-hydrodynamically, and the fate at later time follows owing to the decrease in the force associated with the decay and redistribution of the current. Instead of simulating the evolution of deformation with the magnetic field over a long timescale, we estimate the ellipticity that remains after the formation of the solid crust by comparing two equilibrium models before and after solidification. The relic ellipticity is ~ 10⁻⁸ (B₀/10¹³G)² in the magnitude for dipole field strength B₀ at the surface. This value was primarily determined by the magnetic field at the later time, with a weak dependence on the initial large deformation. The memory of the presolidification state was retained, but was small because the elastic force contributes little to the equilibrium.