Effect of companion mass on neutron-star natal kicks

Determine whether the companion masses in neutron-star binaries affect neutron-star natal-kick velocities sufficiently to explain the difference between the high-kick model for Gaia neutron stars, low-mass X-ray binaries, and neutron-star–white-dwarf binaries and the reduced-kick model for high-mass X-ray binaries and double neutron stars.

Background

The paper finds an apparent bifurcation in the natal kicks inferred for different neutron-star binary populations. Gaia neutron stars, low-mass X-ray binaries, and neutron-star–white-dwarf binaries are broadly consistent with kicks comparable to those of young isolated neutron stars, whereas high-mass X-ray binaries and double neutron stars are better explained by substantially reduced kicks, of order less than 10 km s−1.

The authors identify companion mass as one possible distinguishing factor: high-mass X-ray binaries and double neutron stars generally contain more massive companions than the other systems. Although companion mass influences binary evolution, the paper leaves unresolved whether it also changes the natal-kick velocity through differences such as the stars’ formation or rotational histories.