Supernova Kicks and Misaligned Be Star Binaries

TL;DR

This study examines how different neutron star kick velocity distributions can explain the observed eccentricities and misalignments in Be star/X-ray binary systems, highlighting challenges in reconciling both observations.

Contribution

It investigates the relationship between supernova kicks and binary system properties, proposing that different kick distributions are needed to explain eccentricities and misalignments.

Findings

Low velocity kicks fit eccentricity data but not misalignments.

High velocity kicks fit misalignments but not eccentricities.

Post-supernova orbit circularisation may reconcile high velocity kicks with eccentricity observations.

Abstract

Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a neutron star. They are found to have non-zero eccentricities and there is evidence that some systems have a misalignment between the spin axis of the star and the spin axis of the binary orbit. The eccentricities in these systems are thought to be caused by a kick to the neutron star during the supernova that formed it. Such kicks would also give rise to misalignments. In this paper we investigate the extent to which the same kick distribution can give rise to both the observed eccentricity distribution and the observed misalignments. We find that a Maxwellian distribution of velocity kicks with a low velocity dispersion, $\sigma_k \approx 15\rm km s^{-1}$, is consistent with the observed eccentricity distribution but is hard to reconcile with the observed misalignments, typically $i \ge 25^\circ$. Alternatively a higher velocity kick distribution, $\sigma_k = 265 \rm km s^{-1}$, is consistent with the observed misalignments but not with the observed eccentricities, unless post-supernova circularisation of the binary orbits has taken place. We discuss briefly how this might be achieved.