The X-Ray Orbital Lightcurve of AR Lac

Using the EXOSAT observatory we have obtained the X-ray light curve of the 2 day eclipsing RS CVn binary system AR Lac continuously around one complete binary cycle. A deep minimum centered on primary eclipse is seen in the low energy telescope (0.05-2.0 keV), whereas the ME (1.0-10.0 keV) flux remained constant. This shows that the 5-7 million and the 15-30 million degree components found by Swank et al. (1981) using the Einstein SSS originate in two distinct regions. The failure to detect an eclipse in the ME indicates that the 15-30 million degree plasma comes from a region larger than the radii of the underlying stars. Modelling of the X-ray light curve shows that the lower temperature emission originates from two localized regions on the G star at longitudes of 3° and 150° with azimuthal angular extents of ∼60°. The measured volume can be combined with the RTV scaling relation to infer loop heights of 7,000 km. In addition there is a large structure on the K star that extends from a longitude of 180° to 270° with a height of order one stellar radius; this (probably) contains much hotter plasma with a temperature of 15-20 million degrees.