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Pulsar kick
A pulsar kick is the phenomenon that a neutron star often does not move with the velocity of its progenitor star, but rather with a substantially greater speed. The cause of pulsar kicks is unknown, but many astrophysicists believe that it must be due to an asymmetry in the way a supernova explodes. If true, this would give information about the supernova mechanism.
== Observation ==
It is generally accepted today that the average pulsar kick ranges from 200–500 km/s. However, some pulsars have a much greater velocity. For example, the hypervelocity star B1508+55 has been reported to have a speed of 1100 km/s and a trajectory leading it out of the galaxy. An extremely convincing example of a pulsar kick can be seen in the guitar nebula, where the bow shock generated by the pulsar moving relative to the supernova remnant nebula has been observed and confirms a velocity of 800 km/s.
Of particular interest is whether the magnitude or direction of the pulsar kick has any correlation with other properties of the pulsar, such as the spin axis, magnetic moment, or magnetic field strength. To date, no correlation has been found between the magnetic field strength and the magnitude of the kick. However, there is some contention over whether a correlation between spin axis and kick direction has been observed. For many years, it was believed that no correlation existed. In studies of the Vela and Crab pulsars, jets have been observed which are believed to align with the spin axis of the pulsar. Since these jets align very closely with the bow shock as well as the directly measured velocity of the pulsars, this is considered strong evidence that these pulsars have kicks aligned with their spin axis. It is also possible to measure the spin axis of a pulsar using the polarization of its radiation, and a recent study of 24 pulsars has found a strong correlation between the polarization and kick direction. Such studies have always been fraught with difficulty, however, since uncertainties associated with the polarization measurement are very large, making correlation studies troublesome.
There is a possibility that the distribution of kick speeds is bimodal. Strong evidence for this possibility comes from the "neutron star retention problem". Most globular clusters in the Milky Way have an escape velocity under 50 km/s, so that few pulsars should have any difficulty in escaping. In fact, with the directly measured distribution of kick velocities, we would expect less than 1% of all pulsars born in a globular cluster to remain. But this is not the case—globular clusters contain many pulsars, some in excess of 1000. The number can be improved somewhat if one allows a fraction of the kick momentum to be transferred to a binary partner. In this case, perhaps 6% ought to survive, but this is not sufficient to explain the discrepancy. This appears to imply that some large set of pulsars receive virtually no kick at all while others receive a very large kick. It would be difficult to see this bimodal distribution directly because many speed measurement schemes only put an upper limit on the object's speed. If it is true that some pulsars receive very little kick, this might give us insight into the mechanism for pulsar kicks, since a complete explanation would have to predict this possibility.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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