# An eccentric wave in the circumstellar disc of the Be/X-ray binary X Persei

## TL;DR

This study reports spectroscopic observations of the Be/X-ray binary X Per revealing an eccentric wave in its circumstellar disc, characterized by asymmetric emission lines and a propagating eccentric wave with specific velocity and eccentricity.

## Contribution

It provides the first detailed analysis of an eccentric wave in the circumstellar disc of X Per, including its velocity, eccentricity, and propagation characteristics.

## Key findings

- Eccentric wave appeared first in the innermost disc regions.
- Wave propagates outward with a velocity of approximately 1.1 km/s.
- Eccentricity of the wave is estimated at about 0.29.

## Abstract

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 $H\alpha$, $H\beta$, and HeI 6678 emission lines were symmetric with violet-to-red peak ratio $V/R \approx 1$. 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 $\approx 250$~km~s$^{-1}$ appeared on the red side of the HeI line profile. Later this component emerged in $H\beta$, accompanied by the appearance of a red shoulder in $H\alpha$. 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 $v_{wave} \approx 1.1 \pm 0.2 $~km~s$^{-1}$, and the eccentricity of the eccentric wave is $e_{wave} \approx 0.29 \pm 0.07$. A detailed understanding of the origin of such eccentricities would have applications to a wide range of systems from planetary rings to AGNs.