Optical spectroscopy of the Be/X-ray Binary V850 Cen/GX 304-1 during faint X-ray periodical activity

Be/X-ray binaries are the most populous class of High Mass X-ray Binaries. Their X-ray duty cycle is tightly related to the optical companion wind activity, which in turn can be studied through optical spectroscopical dedicated observations. We study optical spectral features of the Be circumstellar disk to test their long-term variability and their relation with the X-ray activity. Special attention has been given to the H$\alpha$ emission line, that is one of the best tracers of the disk conditions. We obtained optical broadband medium resolution spectra from a dedicated campaign with the Anglo-Australian Telescope and the Southern African Large Telescope in 2014-2015. Data span over one entire binary orbit, and cover both X-ray quiescent and moderately active periods. We used Balmer emission lines to follow the evolution of the circumstellar disk. We observe prominent spectral features, like double-peaked H$\alpha$ and H$\beta$ emission lines. The H$\alpha$ V/R ratio significantly changes over a time scale of about one year. Our observations are consistent with a system observed at a large inclination angle ($i\gtrsim60^{\circ}$). The derived circumstellar disk size shows that the disk evolves from a configuration that prevents accretion onto the neutron star, to one that allows only moderate accretion. This is in agreement with the contemporary observed X-ray activity. Our results are interpreted within the context of inefficient tidal truncation of the circumstellar disk, as expected for this source's binary configuration. We derived the H$\beta$-emitting region size, which results about half of the corresponding H$\alpha$-emitting disk, and constrain the luminosity class of V850 Cen as III-V, consistent with the previously proposed class.}