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The X-ray satellite XMM-Newton has so far revealed coronal cycles in sevensolar-like stars. In this sample, the youngest stars $\epsilon$ Eridani (400Myr) and $\iota$ Horologii (600 Myr) display the shortest X-ray cycles and thesmallest cycle amplitudes. The corona of $\epsilon$ Eridani was modelled interms of solar magnetic structures (active regions, cores of active regions andflares) at varying filling factors. The study revealed that 65-95% of itscorona is covered with magnetic structures, and this was held responsible forthe low X-ray cycle amplitude. It was also hypothesized that the basal surfacecoverage with magnetic structures may be higher on the corona of youngersolar-like stars. To investigate this hypothesis, we study the solar-like starKepler-63 in the X-rays. With an age of 210 Myr and a photospheric cycle of1.27 yr, it is so far the youngest star observed in X-rays with the aim ofrevealing a coronal cycle. In the long-term X-ray lightcurve we do not reveal aperiodic variation of the X-ray luminosity, but a factor two change would bepossible. As for the case of $\epsilon$ Eridani, we modelled the corona ofKepler-63 with magnetic structures observed on the Sun. The study suggests that100% of the corona is composed of cores and flares of Class M, justifying theabsence of an X-ray cycle and confirming the analogous results derived for$\epsilon$ Eridani. Finally, we establish an empirical relation between thecycle amplitude and the X-ray surface flux . From the absence of a coronalcycle in Kepler-63 we infer that stars with higher X-ray flux than Kepler-63must host a significant fraction of higher-energetic flares than thosenecessary to model the corona of Kepler-63. Our study opens new ground forstudies of the solar-stellar analogy and the joint exploration of resolved andunresolved variability in stellar X-ray lightcurves.

The X-ray activity of the young solar-like star Kepler-63 and the structure of its corona