Burial of the polar magnetic field of an accreting neutron star and gravitational wave emission

摘要

We present detailed, ab initio numerical simulations of the distorted magnetic structure of an accreting neutron star as a funsction of accreted mass M<,a>: (i) we evolve an initial dipole through a quasi- static sequence of Grad-Shafranov equilibria while maintaining strict flux-freezing for the first time and (ii) we import these equilibria into the time- dependent, ideal-magnetohydrodynamic (MHD) code ZEUS- 3D and load additional mass on to the star dynanaically. We investigate the stability of these hydromagnetic equilibria by perturbing them in ZEUS- 3D in two and three dimeinsions. Surprisingly, in both cases, the equilibria are marginally stable for all M<,a>≤6×10<'-4>M<,??>. On extending the ZEUS-3D simulations to non-ideal MHD, preliminary results indicate that the equilibria are still not disrupted. The ohmic diffusion time-scale is 10 <'5> (10<'2>) times longer than the accretion time-scale in 2D (3D). The equilibria ("magnetic mountains") are in general, offset from the spin axis and thus generate gravitational waves. We show that the gravitational wave strain is detectable by Advanced LIGO even after Ohmic dissipation and 3D hydromagnetic instabilities are considered.