Constraints on the Steady State r ‐Mode Amplitude in Neutron Star Transients

Abstract

Recent observations suggest that neutron stars in low-mass X-ray binaries rotate within a narrow range of spin frequencies clustered around 300 Hz. A proposed explanation for this remarkable fact is that gravitational radiation from a steady state r-mode oscillation in the neutron star's core halts the spin-up due to accretion. For the neutron star transients, balancing the time-averaged accretion torque with gravitational wave emission from steady state, constant amplitude r-mode pulsations implies a quiescent luminosity too bright to be consistent with observations (in particular of Aql X-1). The viscous dissipation (roughly 10 MeV per accreted nucleon for a spin of 300 Hz) from such an r-mode makes the core sufficiently hot to power a thermal luminosity ∼1034 ergs s-1 when accretion halts. This is the minimum quiescent luminosity that the neutron star must emit when viscous heating in the core is balanced by radiative cooling from the surface, as is the case when the core of the star is superfluid. We therefore conclude that either the accretion torque is much less than Ṁ(GMR)1/2 or a steady state r-mode does not limit the spin rate of the neutron star transients. Future observations with Chandra and XMM promise to further constrain the amount of viscous dissipation in the neutron star core.