Be/X-ray binaries

TL;DR

This review summarizes recent observational findings on Be/X-ray binaries, focusing on the interactions between Be star discs and neutron stars, including disc truncation and its effects on the system's properties.

Contribution

It provides a comprehensive overview of recent observational evidence on the interactions between Be star discs and neutron stars in Be/X-ray binaries.

Findings

Disc truncation leads to smaller, denser discs in binaries.

Interactions influence the Be phenomenon and system evolution.

Recent evidence shows neutron stars significantly affect Be star discs.

Abstract

The purpose of this work is to review the observational properties of Be/X-ray binaries. The open questions in Be/X-ray binaries include those related to the Be star companion, that is, the so-called "Be phenomenon", such as, timescales associated to the formation and dissipation of the equatorial disc, mass-ejection mechanisms, V/R variability, and rotation rates; those related to the neutron star, such as, mass determination, accretion physics, and spin period evolution; but also, those that result from the interaction of the two constituents, such as, disc truncation and mass transfer. Until recently, it was thought that the Be stars' disc was not significantly affected by the neutron star. In this review, I present the observational evidence accumulated in recent years on the interaction between the circumstellar disc and the compact companion. The most obvious effect is the tidal truncation of the disc. As a result, the equatorial discs in Be/X-ray binaries are smaller and denser than those around isolated Be stars.