ScienceWeek

ASTRONOMY: ON MILLISECOND PULSARS

The following points are made by Jonathan E. Grindlay (Science 2006 311:1876):

1) Millisecond pulsars are extreme examples of what can happen when stars evolve into neutron stars in compact binary systems. These rotating objects are spun up by accretion of matter from their binary companions, producing luminous x-ray emission, and later become detectable as pulsars with periods of a few milliseconds [1]. As a result, these "fast pulsars" may offer some of the best probes to study matter and space in the relativistic regime of strong gravity.

2) Hessels et al [2] report the discovery of pulsar PSR J1748-2446ad in the dense globular cluster Terzan 5 (Ter5-ad). This object, detected with the Green Bank radio telescope, holds the new record for the fastest spinning neutron star (or, indeed, any object of stellar mass or larger). Its spin period is only 1.396 ms, even shorter than that of B193721 [the first millisecond pulsar discovered [3]] at 1.558 ms. With a rotation frequency of 716 Hz, Ter5-ad reaches a new high note for the music of the celestial spheres -- between F and F-sharp, whereas B193721 (at 642 Hz) can only hit a note between D-sharp and E.

3) Since their discovery in 1967, pulsars have been the gateway to the study of matter and energy at the extremes found only in neutron stars [4]. Such stars are nature's last stable outposts of matter and are only a factor of ~3 larger in radius than an object that would collapse to a black hole. With ~1.4 to 2 solar masses packed into a radius of ~10 to 15 km, neutron stars are the ultimate laboratories for the astrophysics and physics of the extreme.

4) Neutron stars can exhibit magnetic fields about 10^(15) times that of Earth, as revealed in giant flares from magnetars. And neutron star-binary pairs merge to produce extremely energetic events as revealed in short gamma-ray bursts. However, it is the oldest and fastest pulsars, the millisecond pulsars, that may allow the most direct measures of the ultimate prize: the mass M and radius R of the neutron star itself, which would fix the equation of state and the composition of matter at hypernuclear density. The Ter5-ad system is a new stepping stone on the quest for M and R as well as a constraint on the ultimate rotational limits that may yet be revealed by gravitational waves. A point on the rotation equator of Ter5-ad has velocity nearly one-fourth the speed of light, assuming R = 15 km.[5]

References (abridged):

1. D. Bhattacharya, E. P. J. van den Heuvel, Phys. Rep. 203, 1 (1991)

2. J. W. T. Hessels et al., Science 311, 1901 (2006)

3. D. C. Backer, S. R. Kulkarni, C. Heiles, M. M. Davis, W. M. Goss, Nature 300, 615 (1982)

4. Special Issue on Pusars: Science 304 (23 April 2004).

5. J. M. Lattimer, M. Prakash, Science 304, [536] (2004)

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