Extreme Ultraviolet ExplorerPhotometry of RS Canum Venaticorum Systems: Four Flaring Megaseconds

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Abstract
We present an analysis of 12.2 Ms of Extreme Ultraviolet Explorer (EUVE) photometry of 16 RS CVn systems (including four flaring megaseconds). Our study attempts a systematic categorization of stellar coronal emission in RS CVn binary systems. The temporal resolution of the EUVE satellite and the opportunity for long observations enables us to study separately the quiescent and flaring states of RS CVn systems. Thirty of 31 observations are statistically variable. We examine light curves, characterizing both phase-dependent variations and large- and small-scale flaring. There is evidence for small-scale stochastic variability on the one short-period (Porb0.5Porb and thus cannot originate from compact emission regions close to the stellar surfaces. Many flares have unusual flare morphologies, with rise times comparable to or greater than the decay time, and emission plateaus. In addition, eight flares have decay phases that are fit better by a broken power law than a single power law. The decay times for the second power law are statistically different than the single decay times and are correlated with the flare rise times. This observed connection points to the potential importance of the change in decay rate as a means of examining the physical processes operating during the flare. The distribution of orbital phases of flare onsets on V711 Tau (HR 1099), the best-studied RS CVn in our sample, is consistent with a Poisson distribution that is random in time. In two systems that are partially eclipsing (CF Tuc and ER Vul), we find no evidence for an eclipse in the light curve, indicating that the emission region is large compared to the stellar radius. We parameterize the distribution of flare energies as a power law, finding a cut-off energy of ≈1033 ergs and a slope of -0.6. Flare energy increases with flare duration as E∝Δt1.42, confirming the long-duration nature of high-energy flares on RS CVn systems. The integrated flare luminosity depends on the quiescent luminosity as Lflare∝L1.05quiescent. Flare frequencies range from 0.1 day-1 to 1.5 day-1. For observations not affected by the dead spot on the Deep Survey detector, 40% of the observed time was spent in a flaring state, indicating that flaring is a "normal" state for the coronae of these systems. We note the presence of a quasi-periodicity in EUVE photometric data on the order of 1 day, which is not corrected for in the standard reduction and analysis software.