Constraints on the ubiquity of coronal X-ray cycles

Stellar activity cycles are known to be a widespread phenomenon amongst moderately active solar– and late–type stars from long-term periodic variations in chromospheric Ca ii H and K emission lines, yet to date only a handful of coronal X-ray cycles are known. We have surveyed serendipitously observed stellar sources in fields observed multiple times in the last decade by XMM-Newton and present our analysis of 9 stars from 6 fields. Since our sample is flux–limited, it is strongly biased towards higher levels of X-ray activity. We fit a single temperature apec spectrum to each source and search for significant periodicities using a Lomb–Scargle Periodogram (LSP). We use a Monte Carlo (MC) algorithm to yield robust analysis of the statistical significance of cycle detections and non–detections. None of the 9 stellar lightcurves show any convincing indications of periodicity. From MC simulations we simulate the detection capabilities of our methodology and, assuming a uniform distribution of cycle periods and strengths over the domain searched, we conclude with 95% confidence that less than 72% of the stars represented by our sample of active stars have 5-13 year coronal X-ray cycles. Subject headings: Stars: activity, stars: coronae, X-rays: stars