Spin-perpendicular kicks from evanescent binaries formed in the aftermath of rotational core-collapse and the nature of the observed bimodal distribution of pulsar peculiar velocities

We argue that if core collapse leads to the formation of a rapidly rotating proto-neutron star core or a fizzler surrounded by fall-back material, a lighter proto-neutron star forms around the main star moving in a super-close orbit, as an end result of a fully developed dynamical non-axisymmetric instability. Tidal mass exchange (even through a common envelope phase) propels the lighter star toward the minimum stable mass for a proto-neutron star, whereupon it explodes and the short-lived binary disrupts. The star that remains, a newly born neutron star (or a black hole) acquires a recoil velocity Vkick, according to the law of conservation of linear momentum. A noteworthy feature of this process is that the final kick is determined by nuclear physics, and produces in reality a widespread range in peculiar velocities, up to the highest values observed in the pulsar sample > ∼ 1600 km s. Interestingly, Vkick scales with the mass M of the star that remains as M. The kick, lying in the orbital plane of the binary, is expected to be nearly perpendicular to the spin vector of the post-collapse unstable core, and thus of the neutron star newly formed. Nearly spin-perpendicular kicks of large amplitude are required to explain the observations of geodesic precession in double neutron star binaries such as B1913+16. On the contrary, spin-kick alignment has been claimed for the Vela and Crab pulsars whose transverse speeds are > ∼ 70 and ∼ 170 kms respectively. We suggest that the larger kick component, when present in a pulsar, results from the formation and disruption of an evanescent binary, and is perpendicular to the spin axis; the smaller kick component is associated some other mechanism that leads to less vigorous kicks, predominantly parallel to the spin axis because of phase averaging. This could give rise to a bimodal distribution in the peculiar velocities of neutron stars, as it is observed in the pulsar sample. This scenario may explain the run-away black hole GRO J1655-40, the first to show evidence for a