ASCAObservation of a Long‐Duration X‐Ray Flare from the W UMa–Type Binary VW Cephei

We analyze X-ray archive data of the W UMa-type binary VW Cephei taken with ASCA on 1993 November 5-6. By analyzing the light curve, we find a long-duration flare of ≈ 7.5 hr with a peak luminosity of 1.2 × 10 30 ergs s -1 (0.4-3.0 keV) for the assumed distance of 23.2 pc. A flux dip is detected in the light curve at the orbital phase of ~0.5, and it is identified as an eclipse by the secondary star. We determine the timescale of the eclipse egress to be ~30 minutes from a model fit to the light curve. With this timescale, we estimate that the linear size of the flaring region is ≈ 5.5 × 10 10 cm, regardless of the flare models. From the spectral analysis of the data, we find that the spectrum can be well reproduced by the variable-abundance plasma model with a combination of two different temperatures, kT = 0.64 and kT = 1.91 keV. The hotter component is considered to be associated with the flare. The results are interpreted in terms of a two-ribbon flare model, in which we also discuss the possible enhancement of element abundances.