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

We present an analysis of seven flares detected from five RS CVn-type binaries (UZ Lib, sigma Gem, lambda 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. The exponential decay time in all the sample of flares range from ~ 1 to 8 hrs. The luminosity at peak of the flares in the energy band of 0.3-10.0 keV were found to be in the range of 10^{30.8} - 10^{31.8} erg/s. The great sensitivity of the XMM-EPIC instruments allowed us to perform time resolved spectral analysis during the flares and also in the subsquent quiescent phases. The derived metal abundances of coronal plasma were found to vary during the flares observed from sigma 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 of the order of 10^{10 -11} cm and are smaller than the stellar radii (R*) i.e. L/R* lesssim 1. The flare from sigma Gem showed a high and variable absorption column density during the flare.

A study of X-ray flares - I. Active late-type dwarfs

We present temporal and spectral characteristics of X-ray flares observed from six late-type G-K active dwarfs (V368 Cep, XI Boo, IM Vir, V471 Tau, CC Eri and EP Eri) using data from observations with the XMM-Newton observatory. All the stars were found to be flaring frequently and altogether a total of seventeen flares were detected above the ``quiescent state X-ray emission which varied from 0.5 to 8.3 x 10^{29} erg/s. The largest flare was observed in a low activity dwarf XI Boo with a decay time of 10 ks and ratio of peak flare luminosity to ``quiescent state luminosity of 2. We have studied the spectral changes during the flares by using colour-colour diagram and by detailed spectral analysis during the temporal evolution of the flares. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. We have derived the semiloop lengths of flares based on the hydrodynamic flare model. The size of the flaring loops is found to be less than the stellar radius. The hydrodynamic flare decay analysis indicates the presence of sustained heating during the decay of most flares.