Conditions for Steady Gravitational Radiation from Accreting Neutron Stars

The gravitational wave- and accretion-driven evolution of the angular velocity, core temperature, and (small) amplitude of an r-mode of neutron stars in low-mass X-ray binaries and similar systems is investigated. The conditions required for evolution to a stable equilibrium state (with gravitational wave flux proportional to average X-ray flux) are determined. In keeping with conclusions derived from observations of neutron star cooling, the core neutrons are taken to be normal while the core protons and hyperons and the crust neutrons are taken to be singlet superfluids. The dominant sources of damping are then hyperon bulk viscosity (if much of the core is at least 2-3 times nuclear density) and (e-e and n-n) shear (and possibly magnetic) viscosity within the core-crust boundary layer. It is found that a stable equilibrium state can be reached if the superfluid transition temperature of the hyperons is sufficiently small (≲2 × 109 K), allowing the gravitational radiation from Sco X-1 and several other neutron stars in low-mass X-ray binaries to be potentially detectable by the second-generation LIGO (and VIRGO) arrays.