Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins

Open Access
Publisher Website
Google Scholar
Abstract
We study implications for the apparent alignment of the spin axes, proper motion directions, and polarization vectors of the Crab and Vela pulsars. The spin axes are deduced from recent Chandra X-Ray Observatory images that reveal jets and nebular structure having definite symmetry axes. The alignments indicate that these pulsars were born either in isolation or with negligible velocity contributions from binary motions. We examine the effects of rotation and the conditions under which spin-kick alignment is produced for theoretical models of neutron star kicks. If the kick is generated promptly during the formation of the neutron star by asymmetric mass ejection and/or neutrino emission, then the alignment requires that the proto-neutron star possess, by virtue of the precollapse stellar core's spin, an original spin with period Ps much less than the kick timescale τkick, thus spin averaging the kick forces on the star. The kick timescale ranges from 100 ms to 10 s depending on whether the kick is hydrodynamically driven or neutrino-magnetic field driven. For hydrodynamical models, spin-kick alignment further requires the rotation period of an asymmetry pattern at the radius near shock breakout (100 km) to be much less than τkick 100 ms; this is difficult to satisfy unless rotation plays a dynamically important role in the core collapse and explosion (corresponding to Ps 1 ms). Aligned kick and spin vectors are inherent to the slow process of asymmetric electromagnetic radiation from an off-centered magnetic dipole. We reassess the viability of this electromagnetic rocket effect, correcting a factor of 4 error in Harrison and Tademaru's calculation that increases the size of the effect. To produce a kick velocity of order a few hundred kilometers per second requires that the neutron star be born with Ps ~ 1 ms and that spin-down due to r-mode-driven gravitational radiation be inefficient compared to standard magnetic braking. The electromagnetic rocket operates on a timescale of order 0.3(B/1013 G)-2 yr. The apparent spin-kick alignment in the Crab and Vela pulsars places important new constraints on each of the mechanisms of neutron star kicks that we consider.