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 , 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  as may
be happening in the PSR B1957+20 system , it is not clear whether the energetics or
time-scales for this process are feasible . There is some observational evidence that suggests
that solitary millisecond pulsars are less luminous than binary millisecond pulsars [23, 165]. If
confirmed by future discoveries, this would need to be explained by any viable evolutionary
There are two further “anomalous” isolated pulsars with periods in the range [55, 188].
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 , 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 [150, 91, 42], exactly why so few
are observed is currently not clear.