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

We present results from the high-resolution optical spectroscopy and spectropolarimetry of the peculiar high-mass X-ray binary 4u. 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. 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, both of which yield similar results. We estimate that the outer radius of the Hα-emitting circumstellar disc varies between 7.9 and 15.2 with an average value of 10.9 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, 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 int (V)(%)=0.5 ± 0.2 at the position angle P.A._ int (V) ≈ 47^ ∘