off a nucleus is biased in some direction. So if neutrino emission happened in the presence of a strong magnetic field, we might expect the average neutrino drift to align in some way with that field, and thus the resulting explosion would be asymmetric. A main problem with this theory is that to have sufficient asymmetry the theory requires fields of order 1015 G, much stronger than is expected in a heavy star. Another neutrino based theory uses the fact that the cross section for neutrino scattering depends weakly on the strength of the ambient magnetic field. Thus, if the magnetic field is itself anisotropic, then there could be dark spots which are essentially opaque to neutrinos. This however requires anisotropies of order 1016 G, which is even more unlikely. The final main proposal is known as the electromagnetic rocket scenario. In this theory, we assume the pulsar's magnetic dipole to be offcenter and offaxis from the pulsar's spin axis. This results in an asymmetry in the magnitude of the dipole oscillations, as seen from above and below, which in turn means an asymmetry in the emission of radiation. The radiation pressure then slowly rockets the pulsar away. Notice that this is a postnatal kick, and has nothing to do with asymmetries in the supernova itself. Also notice that this process steals energy from the pulsar's spin, and so a main observational constraint on the theory is the observed rate of rotation for pulsar's throughout the galaxy. A major bonus to this theory is that it actually predicts the spin-kick correlation. However, there is some contention as to whether this can generate sufficient energy to explain the full range of kick velocities. Black hole kicks The large distances above the galactic plane achieved by some binaries are the result of stellar black hole natal kicks. The velocity distribution of black hole natal kicks seems similar to that of neutron-star kick velocities. One might have expected that it would be the momenta that were the same with black holes receiving lower velocity than neutron stars due to their higher mass but that does not seem to be the case. References Bibliography External links Pulsars