A millisecond pulsar is a pulsar with a rotational period in the range of about 1-10 milliseconds. It may be visible in the microwave or X-ray portions of the spectrum.
The origin of millisecond pulsars is still somewhat mysterious. The leading theory is that they begin life as longer period pulsars but are spun up or "recycled" through accretion. For this reason, low-mass X-ray binary systems have received a great deal of attention - they are thought to be pulsars in the process of being recycled.
Such pulsars that emit X-rays are thought to be going through the accretionary stage, actively being sped up. Millisecond pulsars that have already been sped up emit in longer-wavelength portions of the Electromagnetic spectrum.
Current theory of neutron star structure and evolution predicts that pulsars with a period less than one millisecond should not occur. None have yet been detected. Indeed, of the approximately 150 millisecond pulsars so far discovered, the fastest spinning one spins at a rate of 641 revolutions per second, though it radiates in radio wavelengths. At such speeds, it is thought that radiation of gravity waves would act to put the brakes on further acceleration. Indeed, a new discovery of one that spins at 599 revolutions per second and radiates in the X-ray portion of the electromagnetic spectrum IGR J00291+5934 is now a prime candidate for helping detect such waves in the future. Most such pulsars, however, spin at 300 rotations per second.
They are thought to be able to spin at 3000 rotations per second before they break apart, but none so far have been found with anything close to such rotational speeds. This is why gravity wave generation is postulated as a breaking mechanism.
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