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

Abstract

We present spectroscopic observations of the Be/X-ray binary X Per obtained during the period December 2017 -March 2019 (MJD 58095 -MJD 58568). 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 half of the period (MJD 58095 -MJD 58360) the V/R-ratio decreased to 0.5 and the asymmetry developed simultaneously in all three lines. During the second half of the period (MJD 58360 -MJD 58570) the V/R-ratio increased, however with the HeI 6678 line showing a markedly different behaviour than the Balmer lines. A third component with velocity ≥ 250 km s −1 appeared in the red side of HeI line profile. Possibly, during the first half of the period the entire disc become asymmetric and/or elliptical. The third component in HeI can be an indication for inner elliptical disc with eccentricity ∼ 0.4..