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2.2 Pulse periods and slowdown rates

The present public-domain catalogue [208], available on-line [13], contains up-to-date parameters for over 1500 pulsars. Most of these are “normal” with pulse periods P ~ 0.5 s which increase secularly at rates P ~ 10-15 s/s. A growing fraction are “millisecond pulsars”, with 1.5 ms <~ P <~<~ 30 ms and P <~ 10- 19 s/s. For many years, the most rapidly rotating neutron star known was the original millisecond pulsar B1937+21 [19Jump To The Next Citation Point], with P = 1.5578 ms. However, one of the recent discoveries in the globular cluster Terzan 5 appears to be rotating even more rapidly, with P = 1.4 ms [233Jump To The Next Citation Point]. Confirmation of this exciting result should be published in due course. While the hunt for “sub-millisecond pulsars” continues, and most neutron star equations of state allow shorter periods, it has been suggested [36Jump To The Next Citation Point60Jump To The Next Citation Point] that the lack of pulsars with P < 1.5 ms is caused by gravitational wave emission from R-mode instabilities [7Jump To The Next Citation Point].

As can be seen from the “P -P diagram” in Figure 3View Image, normal and millisecond pulsars are distinct populations. The differences in P and P imply fundamentally different magnetic field strengths and ages. Treating the pulsar as a rotating magnetic dipole, one may show [185Jump To The Next Citation Point] that the surface magnetic field strength 1/2 B oc (P P) and the characteristic age tc = P/(2P ).

Lines of constant B and tc are drawn on Figure 3View Image, from which we infer typical values of 12 10 G and 107 yr for the normal pulsars and 108 G and 109 yr for the millisecond pulsars. For the rate of loss of kinetic energy, sometimes called the spin-down luminosity, we have E oc P /P 3. The lines of constant E shown on Figure 3View Image show that the most energetic objects are the very young normal pulsars and the most rapidly spinning millisecond pulsars.

View Image

Figure 3: The P -P diagram showing the current sample of radio pulsars. Binary pulsars are highlighted by open circles. Theoretical models [64] do not predict radio emission outside the dark blue region. Figure provided by Michael Kramer.

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