Pulsar jets: Implications for neutron star kicks and initial spins

We study implications for the apparent alignment of the spin axes, proper m otion directions, and polarization vectors of the Crab and Vela pulsars. Th e spin axes are deduced from recent Chandra X-Ray Observatory images that r eveal jets and nebular structure having definite symmetry axes. The alignme nts indicate that these pulsars were born either in isolation or with negli gible velocity contributions from binary motions. We examine the effects of rotation and the conditions under which spin-kick alignment is produced fo r theoretical models of neutron star kicks. If the kick is generated prompt ly during the formation of the neutron star by asymmetric mass ejection and /or neutrino emission, then the alignment requires that the proto-neutron s tar possess, by virtue of the precollapse stellar core's spin, an original spin with period P-s much less than the kick timescale tau (kick), thus spi n 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 neu trino-magnetic field driven. For hydrodynamical models, spin-kick alignment further requires the rotation period of an asymmetry pattern at the radius near shock breakout (greater than or similar to 100 km) to be much less th an tau (kick) less than or similar to 100 ms; this is difficult to satisfy unless rotation plays a dynamically important role in the core collapse and explosion (corresponding to P-s less than or similar to 1 ms). Aligned kic k and spin vectors are inherent to the slow process of asymmetric electroma gnetic radiation from an off-centered magnetic dipole. We reassess the viab ility of this electromagnetic rocket effect, correcting a factor of 4 error in Harrison and Tademaru's calculation that increases the size of the effe ct. To produce a kick velocity of order a few hundred kilometers per second requires that the neutron star be born with P-s similar to 1 ms and that s pin-down due to r-mode-driven gravitational radiation be inefficient compar ed to standard magnetic braking. The electromagnetic rocket operates on a t imescale of order 0.3(B/10(13) G)(-2) yr. The apparent spin-kick alignment in the Crab and Vela pulsars places important new constraints on each of th e mechanisms of neutron star kicks that we consider.