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2.7 Isolated recycled pulsars

The scenarios outlined qualitatively above represent a reasonable understanding of binary evolution. There are, however, a number of pulsars with spin properties that suggest a phase of recycling took place but have no orbiting companions. While the existence of such systems in globular clusters are more readily explained by the high probability of stellar interactions compared to the disk [283Jump To The Next Citation Point], it is somewhat surprising to find them in the Galactic disk. Out of a total of 66 millisecond pulsars in the Galactic disk, 15 are isolated (see Table 2). Although it has been proposed that these millisecond pulsars have ablated their companion via their strong relativistic winds [158] as may be happening in the PSR B1957+20 system [103Jump To The Next Citation Point], it is not clear whether the energetics or time-scales for this process are feasible [170]. There is some observational evidence that suggests that solitary millisecond pulsars are less luminous than binary millisecond pulsars [23Jump To The Next Citation Point165Jump To The Next Citation Point]. If confirmed by future discoveries, this would need to be explained by any viable evolutionary model.

There are two further “anomalous” isolated pulsars with periods in the range 55-60 ms [55Jump To The Next Citation Point188Jump To The Next Citation Point]. When placed on the P-P diagram, these objects populate the region occupied by the double neutron star binaries. The most natural explanation for their existence, therefore, is that they are “failed double neutron star binaries” which disrupted during the supernova explosion of the secondary [55Jump To The Next Citation Point]. A simple calculation [188Jump To The Next Citation Point], however, suggests that for every double neutron star we should see of order ten such isolated objects. Although some other examples of such pulsars are known [15091Jump To The Next Citation Point42], exactly why so few are observed is currently not clear.


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