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. Because of this theory, systems known as low-mass X-ray binary systems have received a great deal of attention. It is thought that the X-rays in these system are emitted by the accretion disk of a neutron star produced by the outer layers of a companion star that has overflowed its Roche lobe. The transfer of angular momentum from this accretion event can theoretically increase the rotation rate of the pulsar to hundreds of times a second, as is observed in millisecond pulsars.
Many millisecond pulsars are found in globular clusters. This is consistent with the spin-up theory of their formation, as the extremely high stellar density of these clusters implies a much higher likelihood of a pulsar having (or capturing) a giant companion star. Currently there are approximately 130 millisecond pulsars known in globular clusters . The globular cluster Terzan 5 alone contains 33 of these, followed by 47 Tucanae with 22 and M28 and M15 with 8 pulsars each.
The first millisecond pulsar, PSR B1937+21, was discovered in 1982 by Backer et al. Spinning roughly 641 times a second, it remains one of the swiftest-spinning neutron stars of the approximately 180 that have been discovered. Pulsar PSR J1748-2446ad, discovered in 2005, is, as of 2007, the swiftest spinning neutron star currently known, spinning 716 times a second.
Current theories of neutron star structure and evolution predict that pulsars would break apart if they spun at a rate of 1500 rotations per second or more, and that at a rate of above about 1000 rotations per second they would lose energy by gravitational radiation faster than the accretion process would speed them up.
However, in early 2007 data from the Rossi X-ray Timing Explorer and INTEGRAL spacecraft indicated that the neutron star XTE J1739-285 rotates at 1122 Hz.. However, this result is not statistically significant, with a significance level of only 3 sigma. Therefore, while it is an interesting candidate for further observations, current results are only suggestive. Still, it is believed that gravitational radiation plays a role in slowing the rate of rotation. Furthermore, one X-ray pulsar that spins at 599 revolutions per second, IGR J00291+5934, is a prime candidate for helping detect such waves in the future (most such X-ray pulsars only spin at around 300 rotations per second).