Magnetic fields generated by r-modes in accreting millisecond pulsars

Abstract

In rotating neutron stars the existence of the Coriolis force allows the presence of the so-called Rossby oscillations (r-modes) which are known to be unstable to emission of gravitational waves. Here, for the first time, we introduce the magnetic damping rate in the evolution equations of r-modes. We show that r-modes can generate very strong toroidal fields in the core of accreting millisecond pulsars by inducing differential rotation. We shortly discuss the instabilities of the generated magnetic field and its long time-scale evolution in order to clarify how the generated magnetic field can stabilize the star.

Magnetic fields generated by r-modes in accreting quark stars

“…The most relevant one is probably the ambipolar diffusion (see Refs. [39,40]), whose typical timescale is:…”

“…In this paper we extend the calculations of [39,40] to the case of quark stars in LMXB. The motivation for this investigation is that the evolution of accreting stars and their internal magnetic field depends strongly on the r-modes instability window which, for quark stars, is qualitatively different from the one of neutron star.…”

“…R-modes are also responsible for differential rotation in the star which in turn generates a toroidal magnetic field: besides the viscosity of matter, the production of this magnetic field represents a very efficient damping mechanism for the r-modes. This effect has been proposed and investigated in [36][37][38] for the case of a newly born neutron star and it has been recently included within the r-mode equations of the neutron stars in LMXB [39,40]: by calculating the back-reaction of the magnetic field on the r-modes instability, it has been proven that magnetic fields of the order of 10 15 G can be produced. Remarkably, this mechanism could be at the origin of the enormous magnetic field of magnetars.…”