Radio Pulsar Statistics

Forty years after the discovery of radio pulsars by Jocelyn Bell and Antony Hewish at Cambridge in 1967 [22], the observed population presently exceeds 1,700 objects with spin periods in the range 1.4 ms to 8.5 s. Pulsar astronomy is currently enjoying a golden era, with over half of these discoveries in the past 7 years due largely to the phenomenal success of the Parkes multi beam survey [46]. From the sky distribution in Galactic coordinates shown in Fig. 1.1, it is immediately apparent that pulsars are concentrated strongly along the Galactic plane. This is particularly striking for the youngest pulsars known to be associated with supernova remnants. Also shown in Fig. 1.1 are the millisecond pulsars which have spin periods in the range 1.5–30 ms. The more isotropic sky distribution of the millisecond pulsars does not necessarily imply that they have a different spatial distribution; the difference simply reflects the observational bias against detecting short-period pulsars with increasing distance from the Sun. This is one of many selection effects that pervades the observed sample.

From such a violent birth in supernovae, it is perhaps not surprising to learn that pulsars are high-velocity objects. The right-hand panel of Fig. 1.1 shows pulsar proper motions on the plane of the sky taken from a recent study by [23]. The mean transverse speed of the current sample of 233 pulsars is 246 ± 22 km s^-1. From a sample of proper motions for pulsars younger than 3 Myr, Hobbs et al. find the mean 3-D velocity of pulsars to be 400 ± 40 km s^-1. The origin of these high velocities most likely lies in a combination of pre-supernova binary orbital motion (e.g. [24]) and/or impulsive kicks due to small asymmetries in the supernova explosions (e.g. [25]). Millisecond pulsars have significantly lower space velocities; their mean transverse speed is only 87 ± 13 km s^-1, while a study by Lyne et al. [44] showed the mean 3-D speed to be 130 ± 30 km s^-1. Despite these differences, population syntheses indicate that the two populations are consistent with the idea that all neutron stars share the same velocity distribution. The millisecond pulsars represent those binary systems which have survived and have necessarily smaller space velocities as a result [62]