Constraints on the ubiquity of coronal X-ray cycles

TL;DR

This study investigates the prevalence of coronal X-ray cycles in active stars using XMM-Newton data, finding that less than 72% of such stars exhibit 5-13 year cycles, suggesting these cycles are less common than chromospheric activity cycles.

Contribution

The paper provides the first systematic search for long-term coronal X-ray cycles in a flux-limited sample of active stars, using robust statistical methods to constrain their ubiquity.

Findings

No convincing evidence of X-ray cycles in the sample.

Less than 72% of active stars have 5-13 year X-ray cycles with 95% confidence.

Methodology can detect cycles if they are present within the searched parameter space.

Abstract

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.