Evidence from a processing pulsar orbit for a neutron-star birth kick

BIRTH 'kicks' to neutron stars, resulting from asymmetric supernova explosions, have been proposed to explain the high velocities of pulsars(1,2), the existence of companionless, high-velocity massive stars(3,4), and a putative Galactic halo of neutron stars(5). The kick hypothesis has been controversial, because most of the evidence for kicks is indirect, and a physical mechanism to produce asymmetric explosions is as yet unknown(6). Here we report five years of radio observations of the pulsar PSR J0045 - 7319, which is in an eccentric orbit around a B star(7). The data show significant deviations from a simple keplerian orbit, which we interpret as arising from advance of the pulsar's periastron and spin-orbiting coupling(8). Both effects arise because of the B star's rotationally induced equatorial bulge, however spin-orbit coupling requires the B star's spin axis to be inclined with respect to the orbital angular momentum vector; we find that the inclination angle is between 25 and 41 degrees. In the likely event that the angular momenta were aligned before the supernova explosion, this misalignment provides direct evidence that the neutron star received a kick at birth.