Rapid rotation has been recognized as a key prerequisite for significant activity and ensuing large X-ray fluxes; all cool stars follow this rule. Single cool stars universally produce magnetic activity, but early in their youth they are braked through a magnetic wind, hence rapid (single star) rotators must be necessarily young. Another mechanism yielding rapid rotation is tidal coupling, which links the rotation time scale to the orbital time scale in close binary systems. In this fashion, rapid rotation can thus be encountered also in comparatively "old" stars and significant chromospheric and coronal activity is found in such "active binary systems." Here, we consider only "noninteracting" systems in the sense that no (significant) mass accretion through an accretion disk is taking place at least at present.
The RS CVn-type binaries were the first class of stars recognized as a class of stellar X-ray sources even prior to the launch of the Einstein Observatory. RS CVn systems consist of two, usually evolved, but detached late-type stars with orbital periods of a few days up to 2 weeks. These stars are not filling their Roche lobes and in many of the known RS CVn systems the rotation and orbital periods are synchronized. Many if not all of these systems show the usual signatures of extreme activity in the radio, optical, ultraviolet, and X-ray bands. Their X-ray luminosities exceed that of the Sun by up to four orders of magnitude. Another class of X-ray bright close binaries are the Algol systems, consisting of an early-type primary orbited by an evolved lower-mass secondary. Such systems can only be formed by mass transfer and episodical mass transfer may even occur now. However, the observed X-ray activity in Algol systems is thought to be exclusively due to the late-type rapidly rotating companion. WUMa systems consist of two close late-type stars within a common envelope and typical periods of the order of days. Therefore, these systems are rapidly rotating, again leading to large X-ray fluxes with typical X-ray luminosities of up to 1032 erg s-1. Finally, the BY Dra variable stars often consist of two late-type dwarf stars in a relatively close orbit. Although not necessarily tidally coupled, also those objects tend to produce very large X-ray luminosities.
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