Millisecond pulsars with r-modes as steady gravitational radiators

Millisecond pulsars (MSPs) are generally agreed to originate in low-mass X-ray binaries (LMXBs), in which mass transfer onto the neutron stars spins them up to their observed, fast rotation. The lack of MSPs and LMXBs rotating near break-up and the similar rotation periods of several LMXBs have been attributed to the accretion torque being balanced, at fast rotation, by gravitational radiation, perhaps caused by an oscillation mode made unstable through the so-called Chandrasekhar-Friedman-Schutz mechanism. Recently, Wagoner has argued that internal dissipation through weak interaction processes involving $\Lambda^0$ and $\Sigma^-$ hyperons may cause LMXBs to evolve into a quasi-steady state, in which the neutron star has a nearly constant rotation rate, temperature, and mode amplitude. We take this hypothesis one step further, showing that MSPs descending from these LMXBs spend a long time in a similar state, in which a low-amplitude r-mode turns them into extremely steady sources of both gravitational waves and thermal X-rays, while they spin down due to a combination of gravitational radiation and the standard magnetic torque. Observed MSP braking torques already place meaningful constraints on the allowed gravitational wave amplitudes and dissipation mechanisms.