2004ApJS..153..317O

We report on 4 years of multiple wavelength observations of the RS CVn system V711 Tau (HR 1099) from 1993, 1994, 1996, and 1998. This combination of radio, ultraviolet (UV), extreme ultraviolet (EUV), and X-ray observations allows us to view, in the most comprehensive manner currently possible, the coronal and upper atmospheric variability of this active binary system. We report on the changing activity state of the system as recorded in the EUV and radio across the 4 years of observations, and study the high-energy variability using an assemblage of X-ray telescopes. We find the following:

There is evidence for coherent emission at low radio frequencies (≤3 GHz), which appears to be both highly time variable and persistent for several hours. Such phenomena are relatively common, occurring ~30% of the time HR 1099 was observed at L band. The measured polarizations of these bursts are left circularly polarized, in contrast with behavior at higher frequencies, which has the opposite helicity. The polarizations are consistent with a variable source that is 100% left circularly polarized, along with a steady level of flux and polarization, which is 0 or slightly right circularly polarized. There appears to be a low degree of correlation between bursts at 20 cm and higher frequency gyrosynchrotron flares, and also between 20 cm bursts and large EUV/soft X-ray (SXR) outbursts.

Higher frequency (5-8 GHz) flares show an inverse relationship between flux and polarization levels as the flare evolves; this behavior is consistent with flare emission, which is initially unpolarized. Large variations in spectral index are observed, suggesting changes in optical depths of the flaring plasma as the burst progresses. Quiescent polarization spectra show an increase of polarization with frequency, a pattern typically seen in active binary systems but still not understood.

EUV observations reveal several large flares, in addition to numerous smaller enhancements. The total range of variability as gleaned from light curve variations is only a factor of 7, however. Observations in different years provide evidence of a change in the quiescent, not obviously flaring, luminosity, by a factor of up to 2. From an analysis of time-resolved spectral variations, we are able to infer evidence for the creation of high-temperature plasma during flare intervals compared with quiescent intervals. Interpretation of EUV spectral variations is hindered by the lack of ability to diagnose continuum levels and activity-related abundance changes, which are known from higher energy observations. Electron densities determined by line ratios of density-sensitive emission lines are high (10¹²-10¹³/cm3), and there is no evidence for large density enhancements during flare intervals, compared with quiescent intervals.

X-ray observations reveal several flares and provide evidence of energy-dependent flare evolution: harder X-ray energies show faster temporal evolution than at softer energies. Time-resolved X-ray spectral analysis shows the presence of hot plasma, T_e∼30 MK, during flares compared with quiescent intervals, as well as evidence for changing abundances during flares. The abundance of iron (which is subsolar) shows an enhancement of a factor of 3 at the peak of a moderate flare seen by ASCA relative to the preflare level; abundances decrease during the flare decay. No hard (>15 keV) emission is detected by either RXTE or BeppoSAX.

The luminosity ratios L_EUV/L_Rin quiescence determined from several time intervals during the four campaigns are consistent with previously determined ratios from a sample of active stars and solar flares. The range of L_EUV/L_Rfrom three EUV/radio (3.6 cm) flares is the same as the values obtained during quiescence, which points to a common mechanism for producing both flaring and not flaring emission.

Seventeen flares were observed in the EUV and/or SXR during the four campaigns; of the eight flares that had radio coverage, three show 3.6 cm radio flares, which are generally consistent with the Neupert effect. Five EUV/SXR flares had partial UV coverage; all show UV responses, particularly in the C IV transition. The UV flare enhancements can occur at the same time as the 3.6 cm radio flares, in two cases where radio, UV, and EUV/SXR flare coverage overlapped.

For SXR flares, we find that the contrast between flare emission and quiescent emission increases as expected toward higher energies, making flare detections easier at harder X-ray energies. This is due to the creation of high-temperature plasma during flares, which shows up predominantly in high-energy continuum emission. We find a discrepancy between the implied flaring rate based on EUV observations, and higher energy observations; the lower energies tend to miss many of the flares, because of the lack of sufficient contrast with quiescent emission.