2.8 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 [331Jump To The Next Citation Point], it is somewhat surprising to find them in the Galactic disk. Out of a total of 72 Galactic millisecond pulsars, 16 are isolated (see Table 2). Although it has been proposed that these millisecond pulsars have ablated their companion via their strong relativistic winds [194] as may be happening in the PSR B1957+20 system [121Jump To The Next Citation Point], it is not clear whether the energetics or timescales for this process are feasible [211].

There are four further “anomalous” isolated pulsars with periods in the range 28 – 60 ms [59Jump To The Next Citation Point232Jump 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 [59Jump To The Next Citation Point]. A simple calculation [232Jump To The Next Citation Point], suggested that for every double neutron star we should see of order ten such isolated objects. Recent work [30Jump To The Next Citation Point] has investigated why so few are observed. Using the most recent population synthesis models to follow the evolution of binary systems [29Jump To The Next Citation Point], it appears that the discrepancy may not be as significant as previously supposed. In particular, the space velocity distribution of surviving binary systems is narrower than for the isolated objects that were during the second supernova explosion. The isolated systems occupy a larger volume of the Galaxy than the surviving binaries and are harder to detect. When this selection effect is accounted for [30], the relative sample sizes appear to be consistent with the disruption hypothesis.

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