Long-term Optical Observations of the Be/X-ray Binary X Per

TL;DR

This study presents long-term optical spectroscopic observations of X Per, revealing correlations between optical brightness, spectral line variations, and X-ray activity, and estimates the disk viscosity parameter to understand the system's behavior.

Contribution

It provides the first long-term optical spectroscopic dataset of X Per and links mass ejection events to X-ray activity, estimating the disk viscosity parameter.

Findings

Anti-correlations between optical brightness and spectral line intensities during certain epochs.

The viscosity parameter of the circumstellar disk is estimated to be 0.39 and 0.28.

Type I X-ray outbursts are unlikely due to disk truncation at the 2:1 resonance radius.

Abstract

We present the optical spectroscopic observations of X Per from 1999 to 2013 with the 2.16m telescope at Xinglong Station and the 2.4m telescope at Lijiang station, National Astronomical Observatories of China. Combining with the public optical photometric data, we find certain epochs of anti-correlations between the optical brightness and the intensity of the H{\alpha} and HeI 6678 lines, which may be attributed to the mass ejections from the Be star. Alternative explanations are however also possible. The variability of FeII 6317 line in the spectra of X Per might be also caused by the shocked waves formed after the mass ejections from the Be star. The X-ray activities of the system might also be connected with the mass ejection events from the Be star. When the ejected materials were transported from the surface of the Be star to the orbit of neutron star, an X-ray flare could be observed in its X-ray light curves. We use the neutron star as a probe to constrain the motion of the ejected material in the circumstellar disk. With the diffusion time of the ejected material from the surface of Be star to the orbit of neutron star, the viscosity parameter {\alpha} of the circumstellar disk is estimated to be 0.39 and 0.28 for the different time, indicating that the disk around Be star may be truncated by the neutron star at the 2:1 resonance radius and Type I X-ray outburst is unlikely to be observed in X Per.