Evidence for mass-dependent peculiar velocities in compact object binaries: towards better constraints on natal kicks

We compile a catalogue of low-mass and high-mass X-ray binaries, some recently reported binaries that likely host a neutron star (NS) or a black hole (BH), and binary pulsars (a pulsar and a non-degenerated companion) that have measured systemic radial velocities (gamma). Using Gaia and radio proper motions together with., we integrate their Galactic orbits and infer their post-supernova (post-SN) 3D peculiar velocities (v(pec)(z=0) at Galactic plane crossing); these velocities bear imprints of the natal kicks that compact objects received at birth. With the sample totalling 85 objects, we model the overall distribution of v(pec)(z=0) and find a two-component Maxwellian distribution with a low- (sigma(v) approximate to 21 kms(-1)) and a high-velocity (sigma(v) approximate to 107 kms(-1)) component. A further comparison between distributions of binary subgroups suggests that binaries hosting high-mass donors/luminous companions mostly have v(pec)(z=0) less than or similar to 100 kms(-1), while binaries with low-mass companions exhibit a broader distribution that extends up to similar to 400 kms(-1). We also find significant anticorrelations of v(pec)(z=0) with binary total mass (M-tot) and orbital period (P-orb), at over 99 per cent confidence. Specifically, our fit suggests v(pec)(z=0) proportional to M-tot(-0.5) and v(pec)(z=0) proportional to P-orb(-0.2). Discussions are presented on possible interpretation of the correlations in the context of kinematics and possible biases. The sample should enable a range of follow-up studies on compact object binary kinematics and evolution.

Automated summary

The authors compiled a catalogue of X-ray binaries, including binaries with neutron stars or black holes and measured systemic radial velocities. They used Gaia and radio proper motions to integrate the Galactic orbits and infer the post-supernova 3D peculiar velocities. The study found a distribution of velocities with a low-velocity component and a high-velocity component, with significant correlations with binary total mass and orbital period.