The Flaring Corona of UXArietis

Here we present observational results on the RS CVn star UX Arietis from four very-long-baseline interferometry observations distributed in time to cover the rotational period of 6.44 days. The data are better fit by two Gaussian components than by the usual core-plus-halo model. In the first three days the sizes of the two components did not change much from hour to hour but their relative position and orientation changed from day to day. The origin of this evolution can be explained by geometrical factors (i.e., star rotation). The fourth day a large flare occurred and dramatic changes in the sizes of the Gaussian components were seen. RS CVn stars are binary systems characterized by an intense coronal activity at Xray, UV and radio wavelengths. One of the most active sources at radio wavelengths is the system UX Arietis, formed by a G5V and a K0 IV star, with an orbital period of 6.44 days and an orbit diameter of 1.72 mas (Lestrade et al. 1999). Direct evidence of large structures with sizes comparable to the binary system have been provided by VLBI observations (Mutel et al. (1985), Massi et al. (1988), Beasley & Bastian (1996), Franciosini et al. (1999)). The latter, made at a post-flare phase, show a clear variation of the source morphology from day to day. The morphology can be reproduced with a model consisting of two extended elliptical Gaussian components with slowly decaying flux densities, that changed their relative position from day to day. These are interpreted to be two radio loops emerging from the position of the optical spots (see Elias et al. (1995)), intruding into each other. Send offprint requests to: E. Ros Table 1. Observation log Exp. Date Orb. Phase BM140B 2001 Sep 23 0.46 to 0.49 BM140C 2001 Sep 25 0.78 to 0.81 BM140E 2001 Sep 26 0.93 to 0.97 BM140D 2001 Sep 27 0.10 to 0.13 ∗ The array used was the VLBA+Effelsberg in all cases. ⋆ The UT range was 03:30 to 09:30 for all four observing runs. † Observation initially scheduled on Sep 21, but shifted due to scheduling problems in Effelsberg. We performed VLBI observations (see Table 1) with the Very Long Baseline Array (VLBA) and the 100-m Effelsberg telescope during a complete rotation cycle of the system to track the motion of the stellar radio-sphere. We also monitored the total and polarized flux density simultaneously with Effelsberg to distinguish temporal flux density variations from structural variations (see Fig. 1). Activity in UX Arietis was present during all epochs, and a flaring event occurred during the last observing day, Sep 27. 2 Ros & Massi: The Flaring Corona of UX Ari Fig. 1. Flux densities during the four observing runs. Filled boxes represent the single-dish Effelsberg measurements Massi et al. (2005). Empty boxes show the total flux densities obtained from model fitting the visibility data with the a priori calibration from the observing logs (uncertainties 5 %). Stars denote flux densities for the two model fit components (A in dark grey, B in light grey). Since UX Arietis changed in flux density during the observations, we could not image the radio source using the whole data set, since the assumption of an unchanging source, upon which the aperture synthesis principle is based, was violated. We analyzed the data by splitting them in time in 1 hr segments for the first three epochs, and for each observing scan of length 4 min the fourth epoch. The results of this procedure are shown in Fig. 2 and Fig. 3. We interpret our observing results as follows: 1st day; 〈φ〉=0.5: The spotted hemisphere is eclipsed by the K0 IV star itself -the optical spots were not visible except for those at high latitudes (60) 2nd day; 〈φ〉=0.8: the distance between components was maximized, the K0 IV star was to the SW of the GV5 star. 3rd day; 〈φ〉=0.9: the K0 IV star was in the foreground 4th day; 〈φ〉=0.1: The big flare – observations revealed dramatic changes in flux density and size of the model-fit components. Acknowledgements. The VLBA is operated by the National Radio Astronomy Observatory as a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The results presented here are based on observations with the 100-m telescope of the MPIfR (MaxPlanck-Institut fr Radioastronomie) at Effelsberg. Fig. 3. Relative positions in the sky of component B with respect to component A for the four observing epochs (squares: Sep 23; triangles: Sep 25; diamonds: Sep 26; stars: Sep 27).