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

Abstract

If rotating core collapse leads to the formation of a proto-neutron star binary in super-close orbit, then the lighter star, propelled toward the minimum stable mass, explodes. The neutron star (or black hole) that remains acquires a spin-perpendicular kick of very large amplitude. Kicks of the type are required to explain the 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 kicks are relatively small. We propose that the larger kick component, when present in a pulsar, results from the disruption of an evanescent proto-neutron star binary and is spin perpendicular; the smaller kick component is associated to some other mechanism that leads to less vigorous kicks, predominantly parallel to the spin, because of phase averaging. This gives rise to a "bimodal distribution" in the peculiar velocities of neutron stars, as observed in the pulsar sample. The kick velocity is found to scale with the mass of the star that remains as M^(-2/3). This scenario can explain the run-away black hole GRO J1655-40, the first to show evidence for a natal kick.

To appear in the proceedings of the International Workshop on " Pulsars, AXPs, and SGRs observed with BeppoSAX and other Observatories" (Marsala, Sicily, 23-25 September 2002); 6 pages, 1 figure