X-ray stars observed in LAMOST spectral survey

Abstract

X-ray stars have been studied since the beginning of X-ray astronomy. Investigating and studying the chromospheric activity from X-ray stellar optical spectra is highly significant in providing insights into stellar magnetic activity. The big data of LAMOST survey provides an opportunity for researching stellar optical spectroscopic properties of X-ray stars. We inferred the physical properties of X-ray stellar sources from the analysis of LAMOST spectra. First, we cross-matched the X-ray stellar catalogue (12254 X-ray stars) from ARXA with LAMOST data release 3 (DR3), and obtained 984 good spectra from 713 X-ray sources. We then visually inspected and assigned spectral type to each spectrum and calculated the equivalent width (EW) of H $\alpha$ line using the Hammer spectral typing facility. Based on the EW of H $\alpha$ line, we found 203 spectra of 145 X-ray sources with H $\alpha$ emission above the continuum. For these spectra we also measured the EWs of H $\beta$ , H $\gamma$ , H $\delta$ and Ca ii IRT lines of these spectra. After removing novae, planetary nebulae and OB-type stars, we found there are 127 X-ray late-type stars with H $\alpha$ line emission. By using our spectra and results from the literature, we found 53 X-ray stars showing H $\alpha$ variability; these objects are Classical T Tauri stars (CTTs), cataclysmic variables (CVs) or chromospheric activity stars. We also found 18 X-ray stars showing obvious emissions in the Ca ii IRT lines. Of the 18 X-ray stars, 16 are CTTs and 2 are CVs. Finally, we discussed the relationships between the EW of H $\alpha$ line and X-ray flux.