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 [59
]. A simple
calculation [232
], suggested that for every double neutron star we should see of order ten such
isolated objects. Recent work [30
] has investigated why so few are observed. Using the most
recent population synthesis models to follow the evolution of binary systems [29
], 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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