Pulsar velocities

2.10 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. Proper motions for over 250 pulsars have now been measured largely by radio timing and interferometric techniques [240 , 25 , 113 , 138 , 148

, 406 ]. These data imply a broad velocity spectrum ranging from 0 to over 1000 km s^–1 [245

], with the current record holder being PSR B1508+55 [75 ], with a proper motion and parallax measurement implying a transverse velocity of $1083+−19003 km s− 1$ . As Figure 10

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 [245

, 85 ].

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. If the explosion is only slightly asymmetric, an impulsive “kick” velocity of up to 1000 km s^–1 can be imparted to the neutron star [329 ]. 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. The relative contributions of these two factors to the overall pulsar birth velocity distribution is currently not well understood.

The distribution of pulsar velocities has a high velocity component due to the normal pulsars [245 , 148 , 107 ], and a lower velocity component from binary and millisecond pulsars [220 , 84 , 247 , 148 ]. One reason for this dichotomy appears to be that, in order to survive and subsequently form recycled pulsars through the accretion process outlined above, the binary systems contain only those neutron stars with lower birth velocities. In addition, the surviving neutron star has to pull the companion along with it, thus slowing the system down.

Further insights into pulsar kicks from analyses of proper motion and polarization data [168 , 169 , 303 ] find strong evidence for an alignment between the spin axis and the velocity vector at birth. These data have recently been combined with modeling of pulsar-wind nebulae [271 ], where strong evidence is found for a model in which the natal impulse is provided by an anisotropic flux of neutrinos from the proto-neutron star on timescales of a few seconds.