X-ray eclipses in AR Lac: Preliminary results

Observations of the nearby eclipsing RS CVn binary AR Lac using the Imaging Proportional Counter on the Einstein X-ray Satellite have revealed both primary and secondary minima. The depth of these soft X-ray minima are about 30% and 10%, respectively, of the flux at quadrature and the variations are relatively smooth. When compared with observations at other wavelengths the following preliminary conclusions appear to be indicated: (1) The surface flux in soft X-rays from both stars is approximately the same. (2) The soft X-ray corona of the G2 IV-V primary (R1 = 1.8 R/sub theta/) is small. Its depth is no more than 10% of the stellar radius. (3) The soft X-ray corona of the KO IV secondary (R2 = 3.1 R/sub theta/) is large and possibly asymmetric. Apparently the corona extends approximately one stellar radii above the spotted side of this active component. (4) No individual small scale structures (e.g., loops) contain more than 10% of the X-ray flux. This tends to indicate the presence of very large (approx. R/sub */) and/or very many ( 20) loops. (5) Coronal temperatures apparently increase (while densities decrease) with distance from the stellar surfaces. Two consequences are the observed lack of hard X-ray and radio eclipses and the likelihood that the quiescent radio emission in this system is thermal (T approx. 2 x 10 to the eight power K) with gyroresonance absorption playing a key role.