There are four further “anomalous” isolated pulsars with periods in the range 28 – 60 ms [59, 232]. When placed on the 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 . A simple calculation , suggested that for every double neutron star we should see of order ten such isolated objects. Recent work  has investigated why so few are observed. Using the most recent population synthesis models to follow the evolution of binary systems , 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 , the relative sample sizes appear to be consistent with the disruption hypothesis.
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