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

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.

Figures (13)

• Figure 1: Left: Evolution of the Stokes parameters, polarization degree and angle; Top-right: $q-u$ plane. Weighted mean of the source observations (black circles) calculated yearly and of the field stars (red cross); Bottom-right: EDF of measured polarization using all data points (black line) or the weighted averaged points (red line) compared with expected CDF of polarization measurements (blue line). See Sect. \ref{['res:var']} for the meaning of these terms. The data shown correspond to the BeXBs RX J0146.9+6121.
• Figure 2: Two representative examples of the extinction as a function of distance green19. The data points simply mark the assumed location of the stars on the extinction curve, based on their distance but they do not imply any extinction value. Distances are from bailer-jones21. The turquoise circle represents the BeXB, the black circles are the field stars used for the ISM correction, and the blue circles correspond to other field stars. We also indicate the PD of the field stars. The vertical dashed line marks the region of uncertain extinction.
• Figure 3: Polarization degree and angle around the time of a major (type II) X-ray outburst. The vertical dashed lines mark the peak of the X-ray outburst.
• Figure 4: Maximum intrinsic polarization degree as a function of distance (left) and orbital period (right), excluding the outlier EXO 2030+375. The red point corresponds to the Be/$\gamma$-ray binary RX J0240.4+6112/LS I +61303
• Figure 5: Normalize histogram (the $Y$-axis is computed so that the total area under the histogram equals 1) of the maximum intrinsic polarization (corrected for the ISM) of our sample of BeXBs (excluding the outlier EXO 2030+375), compared to the polarization distribution of classical Be stars of similar spectral type yudin01.