"Clocking Millisecond X-Ray Pulsars: A Speed Limit for the Fastest Spinning Stars in the Universe"

Pulsars are highly-magnetized spinning neutron stars formed in the supernova collapse of massive stars at the end of their nuclear-burning lifetime. They are somewhat like giant atomic nuclei, with roughly the Sun's mass compressed into a radius of only 10 kilometers. Some pulsars are spun up to millisecond periods by torques exerted by a binary companion star. These millisecond pulsars are spinning near their centrifugal breakup limit, with surface velocities nearly 20 percent the speed of light. However, recent X-ray timing measurements indicate that the pulsar spin frequency distribution cuts off sharply at the fast end, well before the predicted centrifugal break-up limit is reached. Although the braking mechanism that halts further spin-up is not yet known, these data support theoretical predictions that gravitational radiation losses may be responsible. If so, then these gravitational waves may eventually be detectable by LIGO.