Optical observations of the peculiar high-mass X-ray binary 4U 2206+54

¹ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, Tsarigradsko Chaussee 72, BG-1784 Sofia, Bulgaria
² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland
³ Astrophysikalisches Institut und Universitäts-Sternwarte Jena, Schillergäßchen 2-3, 07745 Jena, Germany
⁴ Byurakan Astrophysical Observatory, Byurakan, 0213 Aragatzotn, Armenia
⁵ Astrophysical Research Laboratory of Physics Institute, Yerevan State University, Yerevan, Armenia

^⋆ Corresponding author: kstoyanov@astro.bas.bg

Received: 25 February 2025
Accepted: 15 April 2025

Abstract

Context. We present results from the high-resolution optical spectroscopy and spectropolarimetry of the peculiar high-mass X-ray binary 4U 2206+54.

Aims. We aim to estimate the size of the circumstellar disc and investigate the interaction between the compact object and the matter in the disc. We also study the interstellar polarisation towards 4U 2206+54.

Methods. We performed a periodogram analysis on the spectral data to identify periodicities. We estimated the radius of the circumstellar disc and the distance between the components at periastron. We used two methods to determine the interstellar polarisation towards 4U 2206+54, both of which yield similar results.

Results. We estimate that the outer radius of the Hα-emitting circumstellar disc varies between 7.9 and 15.2 R_⊙ with an average value of 10.9 R_⊙. This suggests that when the disc is small, the neutron star does not reach the outer parts of the circumstellar disc. Based on our spectropolarimetric observations of 4U 2206+54, we can conclude that the dominant component of the polarisation is interstellar. A depolarisation effect is visible in the Hα emission line, which indicates intrinsic polarisation. We estimate that the intrinsic degree of polarisation in the synthetic V band is P_int (V) (%) = 0.5 ± 0.2 at the position angle PA_int (V)≈47°.