Long-term optical variability of high-mass X-ray binaries

III. Polarimetry

¹ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
² Physics Department, University of Crete, 71003 Heraklion, Greece
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

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Received: 23 January 2026
Accepted: 26 February 2026

Abstract

Context. Systems known as Be/X-ray binaries form 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.

Aims. We present a systematic study of the optical long-term polarimetric variability of Be/X-ray binaries using data collected over ten years. Our aim is to characterize the polarimetric properties of these systems and to probe the structure of their circumstellar disks.

Methods. We monitored Be/X-ray binaries visible from the Northern Hemisphere with the RoboPol polarimeter. We performed a careful analysis of the interstellar polarization in the direction of the sources to estimate their intrinsic polarization. We computed Stokes parameters for linear polarization by aperture photometry and corrected them for instrumental polarization.

Results. 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 more dense than those in Be stars in nonbinary systems. Our data also confirm the presence of highly distorted disks prior to giant X-ray outbursts, although this result is still limited by the lack of simultaneous, well-sampled observations during major X-ray outbursts.