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 binaries (UZ Lib, σ Gem, λ And, V711 Tau and EI Eri) observed with XMM–Newton observatory. The quiescent state X-ray luminosities in the energy band of 0.3–10.0 keV of these stars were found to be 10^30.7-30.9 erg s⁻¹. The exponential decay time in all the sample of flares range from ∼1 to 8 h. The luminosity at peak of the flares in the energy band of 0.3–10.0 keV was found to be in the range of 10^30.8–10^31.8 erg s⁻¹. The great sensitivity of the XMM-European Photon Imaging Camera instrument allowed us to perform time resolved spectral analysis during the flares and also in the subsequent quiescent phases. The derived metal abundances of coronal plasma were found to vary during the flares observed from σ Gem, V771 Tau and EI Eri. In these flares elemental abundances found to be enhanced by factors of ∼1.3–1.5 to the quiescent states. In most of the flares, the peak temperature was found to be more than 100 MK, whereas emission measure increased by factors of 1.5–5.5. Significant sustained heating was present in the majority of flares. The loop lengths (L) derived for flaring structure were found to be in the order of 1010-11 cm and are smaller than the stellar radii (R★), i.e. L/R★≤ 1. The flare from σ Gem showed a high and variable absorption column density during the flare.