A study of X-ray flares - II. RS CVn-type binaries

Abstract

We present an analysis of seven flares detected from five RS CVn-type\nbinaries (UZ Lib, \\sigma Gem, \\lambda And, V711 Tau and EI Eri) observed with\nXMM-Newton observatory. The quiescent state X-ray luminosities in the energy\nband of 0.3-10.0 keV of these stars were found to be 10^{30.7-30.9} erg/s. The\nexponential decay time in all the sample of flares range from ~ 1 to 8 hrs. The\nluminosity at peak of the flares in the energy band of 0.3-10.0 keV were found\nto be in the range of 10^{30.8} - 10^{31.8} erg/s. The great sensitivity of the\nXMM-EPIC instruments allowed us to perform time resolved spectral analysis\nduring the flares and also in the subsquent quiescent phases. The derived metal\nabundances of coronal plasma were found to vary during the flares observed from\n\\sigma Gem, V771 Tau and EI Eri. In these flares elemental abundances found to\nbe enhanced by factors of ~ 1.3-1.5 to the quiescent states. In most of the\nflares, the peak temperature was found to be more than 100 MK whereas emission\nmeasure increased by factors of 1.5 - 5.5. Significant sustained heating was\npresent in the majority of flares. The loop lengths (L) derived for flaring\nstructure were found to be of the order of 10^{10 -11} cm and are smaller than\nthe stellar radii (R*) i.e. L/R* \\lesssim 1. The flare from \\sigma Gem showed a\nhigh and variable absorption column density during the flare.\n