An X-ray outburst from the RS CVn binary HR1099

Abstract

RS CANUM VENATICORUM systems are the most plentiful binary stars known, having a space density of at least 10−6 systems pc−3 (ref. 1). They are detached subgiants of class KO+F to G with orbital periods between 1 day and 2 weeks (ref. 1). Many exhibit wave-like distortions in V which sometimes migrate to earlier binary phase with periods of 10–20 yr. This phenomenon has been interpreted by Hall2 as resulting from extensive star spot activity on the surface of the cooler companion. Steady radio emission has been detected from several RS CVn systems with occasional flares involving an order of magnitude change in flux3–5. More recently, HEAO-A has detected soft X-ray emission (0.2–2.8 keV) from three of these systems HR1099, UX Ari and RS CVn itself6,7, with source luminosities of ∼1030 erg s−1. We report here the detection by the Copernicus 2.5–7.5 keV detector of a 3-h outburst from HR1099, coincident with a radio flare.