X-ray emission of contact binary variables within 1 kpc

Abstract

By assembling the largest sample to date of X-ray emitting EW-type binaries (EWXs), we carried out correlation analyses for the X-ray luminosity log$L_{\text{X}}$, and X-ray activity level log($L_{\text{X}}$/$L_{\text{bol}}$) versus the orbital period $P$ and effective temperature $T_{\mathrm{eff}}$. We find strong $P$-log$L_{\text{X}}$ and $P$-log($L_{\text{X}}$/$L_{\text{bol}}$) correlations for EWXs with $P$ < 0.44 days and we provide the linear parametrizations for these relations, on the basis of which the orbital period can be treated as a good predictor for log$L_{\text{X}}$ and log($L_{\text{X}}$/$L_{\text{bol}}$). The aforementioned binary stellar parameters are all correlated with log$L_{\text{X}}$, while only $T_{\mathrm{eff}}$ exhibits a strong correlation with log($L_{\text{X}}$/$L_{\text{bol}}$). Then, EWXs with higher temperature show lower X-ray activity level, which could indicate the thinning of the convective area related to the magnetic dynamo mechanism. The total X-ray luminosity of an EWX is essentially consistent with that of an X-ray saturated main sequence star with the same mass as its primary, which may imply that the primary star dominates the X-ray emission. The monotonically decreasing $P$-log($L_{\text{X}}$/$L_{\text{bol}}$) relation and the short orbital periods indicate that EWXs could all be in the X-ray saturated state, and they may inherit the changing trend of the saturated X-ray luminosities along with the mass shown by single stars. For EWXs, the orbital period, mass, and effective temperature increase in concordance. We demonstrate that the period $P=0.44$ days corresponds to the primary mass of $\sim 1.1{\mathrm{M}}_{\odot }$, beyond which the saturated X-ray luminosity of single stars will not continue to increase with mass. This explains the break in the positive $P$-log$L_{\text{X}}$ relation for EWXs with $P>0.44$ days.

arXiv comment16 pages, 7 figures and 8 tables; accepted for publication in A&A

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