Millisecond pulsar

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A millisecond pulsar (MSP), often referred to as "recycled 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 electromagnetic 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. 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 [1]. The globular cluster Terzan 5 alone contains 33 of these, followed by 47 Tucanae with 22 and M28 and M15 with 8 pulsars each.

[edit] Pulsar rotational speed limits

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 fastest-spinning neutron star of the approximately 180 that have been discovered; it radiates in radio wavelengths. Pulsar PSR J1748-2446ad, discovered in 2005, is as of 2006 the fastest spinning neutron star currently known, spinning 716 times a second [2], [3].

Current theories of neutron star structure and evolution predict the following limits to the rotational speed of pulsars:

they are thought not to be able to pass 1500 rotations per second lest they break apart;
and well before such a high rotational speed is reached, it is thought that radiation of gravitational waves would act to put the brakes on further acceleration, in effect preventing rotational speeds above 1000 rotations per second (corresponding to periods of less than one millisecond).

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.^[1] 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).