Two decades of optical variability of Small Magellanic Cloud high-mass X-ray binaries

TL;DR

This study analyzes 20 years of optical/IR data from 111 high-mass X-ray binaries in the Small Magellanic Cloud, revealing complex variability patterns and proposing a new classification linking optical behavior to X-ray properties.

Contribution

It introduces a new taxonomy for super-orbital variability in HMXBs and refines orbital period estimates, enhancing understanding of their optical and X-ray interactions.

Findings

Most systems show variability across multiple time scales

A new taxonomy links optical variability to X-ray behavior

Refined orbital periods and identified sub-orbital variability

Abstract

We present an analysis of the long-term optical/IR behavior of 111 high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud based on data from the OGLE collaboration. Most systems exhibit variability on a range of time scales. This variability regulates the mass transfer to the compact object, while the compact object can, in turn, affect the donor star's behavior. To better understand this complex interaction and the resulting X-ray properties in these systems, we define a new taxonomy for the observed super-orbital variability. This taxonomy connects to the color changes, orbital periods, and X-ray behavior of the sources. In most cases, these properties can be explained by differences between the flux of the disk around the Be star and the flux from the star itself. We also refine and present new potential orbital periods and sub-orbital variability in the sources.