Arresting Accretion Torques with Gravitational Radiation

Abstract

Recent theoretical work has made it plausible for neutron stars (NSs) to lose angular momentum via gravitational radiation on long timescales (around Myrs) while actively accreting. The gravitational waves (GWs) can either be emitted via the excitation of r-modes or from a deformed crust. GW emission can thus intervene to slow-down or halt the otherwise relentless spin-up from accretion. Prior to this theoretical work (and the measurements of NS rotation rates in LMXBs) the community was rather confident that an accreting NS would be spun-up to rotation rates near breakup, motivating searches for sub-millisecond objects. After only briefly describing the physics of the GW processes, I argue that the limiting spin frequency might be appreciably lower than the breakup frequency. Millisecond radio pulsar observers would likely discover the impact of GW emission as a dropoff in the number of pulsars beyond 600 Hz, and I show here that the millisecond pulsar inventory in 47 Tuc might already exhibit such a cutoff. These theoretical ideas will be tested by GW searches with ground-based interferometers, such as the advanced LIGO instrument proposed for operation by 2008.