Rotation-powered millisecond pulsars represent a distinct population of pulsars, characterized by incredibly rapid rotation rates (as fast as 716 rotations per second). These neutron stars are widely believed to be the end-products of an exotic channel of binary evolution, involving an extended period of accretion of matter and angular momentum from a close stellar companion in X-ray binary systems. After a period of 10.100 million years, they acquire millisecond spin periods and after accretion ceases are reactivated ("recycled") as radio pulsars. This theory is supported by the fact that 80% of the known millisecond pulsars in the Galactic disk are found in binary systems, whereas this is the case for only 1% of the general pulsar population. In addition, a multitude of millisecond pulsars are found in the cores of globular clusters where the occurrence of compact binaries is much higher than in the field of the Galaxy.
Most of the observable radiation from millisecond pulsars is expected in the X-ray band. In recent years, the unprecedented sensitivity and spatial resolution of the Chandra X-Ray Observatory have allowed the detection and study of numerous millisecond pulsars in several globular clusters (19 in 47 Tuc alone). In addition, the Chandra and XMM-Newton telescopes have enabled much more detailed spectroscopic and timing studies of the existing sample of X-ray-detected millisecond pulsars than were previously possible. These studies have revealed that the majority of these objects are seen in X-rays due to their hot (million degree) polar caps. This thermal radiation provides an excellent probe of the extreme environment near the surface of the neutron star. Specifically,
it provides unique insight into the surface properties, magnetic field of the star, and measure the mass and radius of the star. Knowledge of the mass and radius of a neutron star, in particular, is of tremendous importance in astrophysics as it provides insight into the behavior of matter at supra-nuclear densities. Since such extreme conditions are currently not producible in terrestrial laboratories, neutron stars serve as valuable natural laboratories for studies of extreme physics.
For more details, see:
"Constraints on Neutron Star Properties from X-ray Observations of Rotation-powered Millisecond Pulsars" Bogdanov, S., Rybicki, G. B. & Grindlay, J. E. 2007, ApJ, submitted (astro-ph/0612791)
"Chandra X-Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae" Bogdanov, S., Grindlay, J. E., Heinke, C. O., Camilo, F., Freire, P. C. C., Becker, W. 2006, ApJ, 646, 1104