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. 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. It is possible that they are neutron stars that have been spun-up to spin hundreds of times a second by the transfer of angular momentum by a companion star that has overflowed its Roche lobe. Millisecond pulsars that have already been sped up emit in longer-wavelength portions of the Electromagnetic spectrum.

Many millisecond pulsars are found in globular clusters because the extremely high stellar density of these systems leads to exchange interactions that create the kind of mass-transfer binaries that spin pulsars up to millisecond pulsars. 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).

Indeed, no “sub-millisecond pulsars” have yet been detected as of 2006 (the fastest known rotates at 716 Hz, as mentioned above). This suggests that loss of energy through gravity wave generation is indeed the braking mechanism in action in the fastest spinning neutron stars. Consequently, a new discovery of one X-ray pulsar that spins at 599 revolutions per second, IGR J00291+5934, is now a prime candidate for helping detect such waves in the future (most such X-ray pulsars only spin at around 300 rotations per second).