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2.8 Pulsar velocities

Pulsars have long been known to have space velocities at least an order of magnitude larger than those of their main sequence progenitors, which have typical values between 10 and 50 km s-1. The first direct evidence for large velocities came from optical observations of the Crab pulsar [321], showing that the neutron star has a velocity in excess of -1 100 km s. Proper motions for 233 pulsars have subsequently been measured largely by radio timing and interferometric techniques [1942496115122Jump To The Next Citation Point]. These data imply a broad velocity spectrum ranging from 0 to over 1000 km s- 1 [201Jump To The Next Citation Point].
Watch/download Movie

Figure 10: GIF movie showing a simulation following the motion of 100 pulsars in a model gravitational potential of our Galaxy for 200 Myr. The view is edge-on, i.e. the horizontal axis represents the Galactic plane (30 kpc across) while the vertical axis represents ±10 kpc from the plane. This snapshot shows the initial configuration of young neutron stars.
Such large velocities are perhaps not surprising, given the violent conditions under which neutron stars are formed. Shklovskii [282] demonstrated that, if the explosion is only slightly asymmetric, an impulsive “kick” velocity of up to -1 1000 km s can be imparted to the neutron star. In addition, if the neutron star progenitor was a member of a binary system prior to the explosion, the pre-supernova orbital velocity will also contribute to the resulting speed of the newly-formed pulsar. As Figure 10Watch/download Movie illustrates, high-velocity pulsars born close to the Galactic plane quickly migrate to higher Galactic latitudes. Given such a broad velocity spectrum, as many as half of all pulsars will eventually escape the Galactic gravitational potential [20171].

The distribution of pulsar velocities remains an issue of contention. For example, Monte Carlo simulations by Arzoumanian et al. [8Jump To The Next Citation Point] strongly favour a bimodal distribution with low and high velocity peaks. On the other hand, a recent study [122] found the mean birth velocity of normal pulsars to be consistent with a Maxwellian distribution with a mean of -1 ~ 400 km s. Regardless of the form of the distribution, however, we can say that the mean velocity of young pulsars is significantly larger than the 80- 140 km s- 1 range for millisecond and binary pulsars [17870205Jump To The Next Citation Point]. The main reasons for the lower velocities are the fact that the kick must have been small to avoid disruption, and the surviving neutron star has to pull the companion along with it, thus slowing the system down.


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