We present spectroscopic observations of the Be/X-ray binary X Per obtained during the period December 2017 - January 2020 (MJD~58095 - MJD~58865). In December 2017 the Hα, Hβ, and HeI 6678 emission lines were symmetric with violet-to-red peak ratio V/R≈1. During the first part of the period (December 2017 - August 2018) the V/R-ratio decreased to 0.5 and the asymmetry developed simultaneously in all three lines. In September 2018, a third component with velocity ≈250~km~s−1 appeared on the red side of the HeI line profile. Later this component emerged in Hβ, accompanied by the appearance of a red shoulder in Hα. Assuming that it is due to an eccentric wave in the circumstellar disc, we find that the eccentric wave appeared first in the innermost part of the disc, it spreads out with outflowing velocity vwave≈1.1±0.2~km~s−1, and the eccentricity of the eccentric wave is ewave≈0.29±0.07. A detailed understanding of the origin of such eccentricities would have applications to a wide range of systems from planetary rings to AGNs.
@article{arxiv.1906.11587,
title = {An eccentric wave in the circumstellar disc of the Be/X-ray binary X Persei},
author = {R. K. Zamanov and K. A. Stoyanov and U. Wolter and D. Marchev and N. A. Tomov and M. F. Bode and Y. M. Nikolov and V. Marchev and L. Iliev and I. K. Stateva},
journal= {arXiv preprint arXiv:1906.11587},
year = {2020}
}