Landscape of Coronal X-Ray Variability and Cycles

Abstract Coronal (1–10 MK) X-rays display dramatic variability over the Sun’s iconic 11 yr magnetic dynamo cycle: already a factor of 4 in the soft 0.1–2.4 keV “ROSAT band,” soaring to more than 100 at harder energies (>10 keV). The high-energy variations impact heliospheric space weather (SW); presumably likewise for host-star analogs. In an effort to better document long-term coronal variability and X-ray cycles of other stars, measurements of 19 late-type F–M dwarfs and subgiants were obtained from archives of the three contemporary long-lived X-ray observatories: Chandra, XMM-Newton, and Swift. The X-ray event lists were time-filtered to suppress transients like flares and telemetry dropouts. A novel scheme, based on empirical coronal models, harmonized flux conversions across the different instruments. The Sun was included based on high-energy irradiance time series. Results generally confirmed previous findings: high-contrast, decadal-class X-ray modulations were found exclusively at low-to-medium L X / L BOL ; higher X-ray intensity stars displayed lower-amplitude, faster variations, if cycling at all; whereas the highest activity classes showed stable (“saturated”) long-term X-ray trends, but punctuated by persistent flaring. In addition, several variants of “dynamo diagrams” are presented to illustrate possible correlations among key parameters, such as rotation period and cycle duration. Early versions of such diagrams had displayed what appeared to be clear trends, although additional observations in recent years have tended to downplay the previous relationships. The diverse X-ray behaviors hold implications for stellar SW, as well as posing tough challenges for dynamo theory.