X-ray spectroscopy of the W UMa-type binary VW Cephei

Abstract

VW Cephei, a W UMa-type binary system, was observed twice with a four week interval in October 2002 by the XMM-Newton space observatory. During the first observation, the source appears to be quiescent. A shallow dip in the X-ray light curve during primary eclipse indicates that the primary and secondary stars both contribute to the X-ray emission which could arise from an extended corona encompassing the two companions. Spectral fitting of the EPIC spectra suggests a corona configuration with little contribution from quiet regions, similar to the Sun. On the contrary, the 0.2–0.8 keV temperature of the “cool” plasma components is reminiscent of solar- type active regions, while the hot ($T \ge$ 1 keV) component may be caused by disruptions of magnetic fields associated with flaring activity. During the second observation, a large flare occured in the corona above the primary component around the time of primary eclipse. The VW Cep count rate decayed by 30% in about 3.7 ksec and the emission measure of hot ($T > 10^{7}$ K) plasma varied by a factor of 4. The flare was comparable in size to two-ribbon flares observed on the Sun. Intense flaring activity on VW Cep is supported by a neon abundance enhancement relative to oxygen reminiscent of abundance anomalies observed during solar and stellar flares. Compared with other active binary systems such as RS CVn or BY Dra, VW Cep has relatively less material at temperatures higher than 10 7 K and the temperatures of hot plasmas appear to be lower.