X-ray activity cycle on the active ultra-fast rotator AB Doradus A?. Implication of correlated coronal and photometric variability

Context. Although chromospheric activity cycles have been studied in a larger number of late-type stars for quite some time, very little is known about coronal activity-cycles in other stars and their similarities or dissimilarities with the solar activity cycle.
Aims: While it is usually assumed that cyclic activity is present only in stars of low to moderate activity, we investigate whether the ultra-fast rotator AB Dor, a K dwarf exhibiting signs of substantial magnetic activity in essentially all wavelength bands, exhibits an X-ray activity cycle in analogy to its photospheric activity cycle of about 17 years and possible correlations between these bands.
Methods: We analysed the combined optical photometric data of AB Dor A, which span ~35 years. Additionally, we used ROSAT and XMM-Newton X-ray observations of AB Dor A to study the long-term evolution of magnetic activity in this active K dwarf over nearly three decades and searched for X-ray activity cycles and related photometric brightness changes.
Results: AB Dor A exhibits photometric brightness variations ranging between 6.75 < V_mag ≤ 7.15 while the X-ray luminosities range between 29.8 < log L_X [erg/s] ≤ 30.2 in the 0.3-2.5 keV. As a very active star, AB Dor A shows frequent X-ray flaring, but in the long XMM-Newton observations a kind of basal state is attained very often. This basal state probably varies with the photospheric activity-cycle of AB Dor A, which has a period of ~17 years, but the X-ray variability amounts at most to a factor of ~2, which is, much lower than the typical cycle amplitudes found on the Sun.