R-modes of accreting hyperon stars as persistent sources of gravitational waves

Abstract

The $r$-modes of accreting neutron stars could be a detectable source of persistent gravitational waves if the bulk viscosity of the stellar matter can prevent a thermal runaway. This is possible if exotic particles such as hyperons are present in the core of the star. We compute bulk viscous damping rates and critical frequencies for $r$-modes of neutron stars containing hyperons in the framework of relativistic mean field theory. We combine the results of several previous calculations of the microphysics, include for the first time the effect of rotation, and explore the effects of various parameters on the viability of persistent gravitational wave emission. We find that persistent emission is quite robust, although it is disfavored in stars below 1.3--1.5 $M_\odot$ depending on the equation of state. In some cases persistent emission is compatible with temperatures as low as $10^7$ K, observed in some accreting neutron stars in quiescence.