X-ray emission of contact binary variables within 1 kpc
Esamdin, Ali; Gu, Wei-Min; Liu, Junhui; Wang, Junfeng; Wu, Jianfeng; Sun, Mouyuan
China
Abstract
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 LX, and X-ray activity level log(LX/Lbol) versus the orbital period, P, effective temperature, Teff, 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 LX and P-log(LX/Lbol) 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 LX and log(LX/Lbol). The aforementioned binary stellar parameters are all correlated with log LX, while only Teff exhibits a strong correlation with log(LX/Lbol). 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(LX/Lbol) 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 LX relation for EWXs with P > 0.44 days.