A distinct population
The millisecond pulsars are in a different category from the general population of ‘normal’ pulsars. The majority of pulsars are following a simple course of evolution, from a birth in a supernova, through a slowdown from a rotational period at birth of some tens of milliseconds to a death at around one second when the radiation ceases or becomes undetectable. The millisecond pulsars constitute a separate and much longer-lived population, which originate from the general population of normal pulsars as the result of gravitational interactions with binary partners; most still have their companions, but some have lost them or are possibly in the process of losing them, thus becoming solitary millisecond pulsars.
Most millisecond pulsars have rotational periods less than 10 ms, although a more useful definition includes periods up to 30 ms. Figure 11.1 shows their position in the P/Ṗ diagram, which also shows all known binary systems as circles. Although there is a distinct gap between normal and binary pulsars in this diagram, there are some pulsars within the gap; most of these have the characteristics of millisecond pulsars such as a comparatively low slowdown rate, and they have probably experienced the same binary interactions.
It is remarkable that almost all of the neutron stars associated with X-ray sources are members of binary systems, while only one in ten of the radio pulsars is a binary.