X-ray emission of contact binary variables within 1 kpc

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, logL 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-logL 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 logL X and log(L X /L bol ). The aforementioned binary stellar parameters are all correlated with logL 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 ∼ 1.1M , 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-logL X relation for EWXs with P > 0.44 days.

A Study of Twelve Potential Merger Candidate Contact Binary Systems

“…Coronal and chromospheric activity combined with fast synchronous rotation of the common envelopes may also result in X-Ray emissions from contact binaries (Gondoin 2004;Kandulapati et al 2015). Seven of our systems (A0458, A0514, V396 Lup, A1846, A2044, S2213, A2348) were reported as potential X-Ray sources (Liu et al 2022) including three (A0514, V396 Lup and A2044) without FUV observations bringing to a total of nine systems with signs suggestive of increased chromospheric activity without characteristic photospheric changes.…”

“…Another high energy feature of excess chromospheric/magnetic activity is X-Ray emissions (Gondoin 2004). Only a small fraction of contact binary star systems are X-ray sources (Liu et al 2022) such that presence of X-Ray emissions can be taken as a marker of increased magnetic activity and potential indicator for an increased loss of angular momentum and orbital instability. Seven of twelve systems reported have been cataloged as having X-Ray emissions (Liu et al 2022).…”

“…Only a small fraction of contact binary star systems are X-ray sources (Liu et al 2022) such that presence of X-Ray emissions can be taken as a marker of increased magnetic activity and potential indicator for an increased loss of angular momentum and orbital instability. Seven of twelve systems reported have been cataloged as having X-Ray emissions (Liu et al 2022). Also, as noted above the FUV magnitude can be used as a marker for chromospheric activity.…”

The X-Ray Emission Reveals the Coronal Activities of Semi-detached Binaries

“…The X-ray emission can act as a proxy for the efficiency of the stellar dynamo (Magaudda et al 2022) and serve as a manifestation of magnetic activity in the outermost atmospheric layer, namely, the corona. The stellar dynamo magnetic activity produced by fast rotation and envelope convection, as well as the large-scale horizontal flow between the components, are considered to be the possible X-ray emission mechanism of EW-type close binaries (e.g., Stȩpień et al 2001;Gondoin 2004;Chen et al 2006;Liu et al 2022). However, there is currently limited statistical research on the relationship between the X-ray emission properties of EBXs and the stellar dynamo model.…”

“…However, there is currently limited statistical research on the relationship between the X-ray emission properties of EBXs and the stellar dynamo model. Moreover, for late-type main-sequence stars and close binaries, the ratio of X-ray luminosity L X to bolometric luminosity L bol is used to represent the magnetic activity level of an individual system (Fleming et al 1993;Güdel 2004;Chen et al 2006;Liu et al 2022). This ratio tends to reach a maximum level at 10 −3 , i.e., log(L X /L bol ) ∼ −3, which is called the saturation limit or saturation level (Vilhu 1984;Vilhu & Walter 1987;Fleming et al 1993).…”