Long-term optical variability of high-mass X-ray binaries. III. Polarimetry

TL;DR

This study systematically analyzes a decade of optical polarimetric data of Be/X-ray binaries to understand their circumstellar disk structures and variability.

Contribution

It provides the first long-term polarimetric characterization of Be/X-ray binaries, confirming smaller, denser disks compared to single Be stars and linking disk distortion to X-ray outbursts.

Findings

Optical polarimetric variability is common in Be/X-ray binaries.

Disks in BeXBs are smaller and denser than in single Be stars.

Highly distorted disks are observed before giant X-ray outbursts.

Abstract

Be/X-ray binaries are the most numerous group of high-mass X-ray binaries. Their long-term optical and infrared variability reflects the evolution of the circumstellar disk around the luminous companion. This variability manifests photometrically as an excess of flux that increases with wavelength and spectroscopically as line emission. The disk is also expected to generate linear polarization. We present a systematic study of the optical long-term polarimetric variability of Be/X-ray binaries on data collected over 10 years. Our aim is to characterize the polarimetric properties of these systems and to probe the structure of their circumstellar disks. We have been monitoring Be/X-ray binaries visible from the Northern hemisphere with the RoboPol polarimeter. Optical polarimetric variability is a common trait in Be/X-ray binaries. The variability can be attributed to the Be star's circumstellar disk. Our polarization analysis confirms previous claims based on spectroscopic data that the circumstellar disks in BeXBs are, on average, smaller and denser than those in Be stars in non-binary systems. Our data also confirms the presence of highly distorted disks before giant X-ray outbursts, although this result is still affected by the lack of simultaneous and well-sampled observations during major X-ray outbursts.