2022A&A...663A.115L

Aims. The X-ray emission of contact binaries (EW-type) is an important facet of such systems. Thus, X-ray emitting EW-type binaries (EWXs) are ideal laboratories for studying the X-ray radiation saturation mechanisms as well as binary evolution. By assembling the largest sample to date of EWXs with periods of less than 1 day from the All-Sky Automated Survey for Supernovae Variable Stars Database and X-ray catalogs from the XMM-Newton and ROSAT missions, we aim to conduct a systematic population study of X-ray emission properties of EWXs within 1 kpc.
Methods. We carried out correlation analyses for the X-ray luminosity, log L_X, and X-ray activity level log(L_X/L_bol) versus the orbital period, P, effective temperature, T_eff, metallicity [Fe/H], and the surface gravity log g of EWXs. We investigated the relation between X-ray emission and the mass of component stars in the binary systems. We also performed sample simulations to explore the degeneracy between period, mass, and effective temperature for EWXs.
Results. We find strong P-log L_X and P-log(L_X/L_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_X and log(L_X/L_bol). The aforementioned binary stellar parameters are all correlated with log L_X, while only T_eff exhibits a strong correlation with log(L_X/L_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_X/L_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 ∼1.1 M_☉, 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_X relation for EWXs with P > 0.44 days.