Polar kicks and the spin period - eccentricity relation in double neutron stars

TL;DR

This study investigates how spin-kick alignment influences the correlation between neutron star spin periods and orbital eccentricities in double neutron star systems, suggesting different formation channels for varying eccentricities.

Contribution

It demonstrates that spin-kick alignment can explain observed correlations without requiring low kick velocities, and proposes distinct formation pathways based on mass transfer history.

Findings

Spin-kick alignment is compatible with observed correlations.

Low- and high-eccentricity systems likely form through different channels.

Eccentric systems may indicate neutron star accretion during giant companion envelopes.

Abstract

We present results of a population synthesis study aimed at examining the role of spin-kick alignment in producing a correlation between the spin period of the first-born neutron star and the orbital eccentricity of observed double neutron star binaries in the Galactic disk. We find spin-kick alignment to be compatible with the observed correlation, but not to alleviate the requirements for low kick velocities suggested in previous population synthesis studies. Our results furthermore suggest low- and high-eccentricity systems may form through two distinct formation channels distinguished by the presence or absence of a stable mass transfer phase before the formation of the second neutron star. The presence of highly eccentric systems in the observed sample of double neutron stars may furthermore support the notion that neutron stars accrete matter when moving through the envelope of a giant companion.