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D. Lai, D. F. Chernoff, J. M. Cordes (Cornell University)
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 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 protoneutron star possess, by virtue of the precollapse stellar core's spin, an original spin with period Ps much less than the kick timescale \tau\rm kick, thus spin-averaging the kick forces on the star. The kick timescale ranges from 100~ms to 10 seconds 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 \tau\rm kick< 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 km~s-1 requires that the neutron star be born with Ps ~1 ms and that spindown due to r-mode driven gravitational radiation be inefficient compared to standard magnetic braking. The apparent spin-kick alignment in the Crab and Vela pulsars places important new constraints on each of the mechanisms of neutron star kicks we consider.
This work is supported in part by NASA grants NAG 5-8356 and NAG 5-8484, by NSF grants AST 9819931 and AST 9986740, and by a research fellowship from the Alfred P. Sloan foundation.
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