LONG-TERM OPTICAL OBSERVATIONS OF THE Be/X-RAY BINARY X PER

     Be/X-ray binary X Per has an orbital period of 250d with an eccentricity of 0.11.  We present optical spectroscopic observations of X Per from 1999 to 2013 with the 2.16 m telescope at Xinglong Station and the 2.4 m telescope at Lijiang Station, National Astronomical Observatories of China. Combining these observations with the public optical photometric data, we find certain epochs of anti-correlations between the optical brightness and the intensity of the Hα and He I 6678 lines (see three regions separated with dashed lines in the figure), which may be attributed to 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 the 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 the Be star to the orbit of neutron star, the viscosity parameter α of the circumstellar disk is estimated to be 0.39 and 0.28 for the different times, indicating that the disk around the Be star may be truncated by the neutron star at the 2:1 resonance radius and that a Type I X-ray outburst is unlikely to be observed in X Per.

    In this work, we present the simultaneous visual photometric, spectroscopic, and X-ray observations in Section 2. We then analyze the variability of the Hα, Hei 6678, and Fe ii 6317 lines in our spectra together with the long-term V-band light curves and the activities of the X-ray emission of the system in Section 3. In Section 4, we discuss the relationship between the optical brightness and the intensity of the emission lines. We also study the correlation between the optical emission and the activities of the X-ray emission. At last, we give a summary of the main findings in Section 5.

This work is done by Hui Li1,Jingzhi Yan, Jianeng Zhou, and Qingzhong Liu, is available at http://iopscience.iop.org/1538-3881/148/6/113/pdf/1538-3881_148_6_113.pdf