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

Abstract

The X-ray satellite XMM-Newton has so far revealed coronal cycles in seven solar-like stars. In this sample, the youngest stars $\epsilon$ Eridani (400 Myr) and $\iota$ Horologii (600 Myr) display the shortest X-ray cycles and the smallest cycle amplitudes. The corona of $\epsilon$ Eridani was modelled in terms of solar magnetic structures (active regions, cores of active regions and flares) at varying filling factors. The study revealed that 65-95% of its corona is covered with magnetic structures, and this was held responsible for the low X-ray cycle amplitude. It was also hypothesized that the basal surface coverage with magnetic structures may be higher on the corona of younger solar-like stars. To investigate this hypothesis, we study the solar-like star Kepler-63 in the X-rays. With an age of 210 Myr and a photospheric cycle of 1.27 yr, it is so far the youngest star observed in X-rays with the aim of revealing a coronal cycle. In the long-term X-ray lightcurve we do not reveal a periodic variation of the X-ray luminosity, but a factor two change would be possible. As for the case of $\epsilon$ Eridani, we modelled the corona of Kepler-63 with magnetic structures observed on the Sun. The study suggests that 100% of the corona is composed of cores and flares of Class M, justifying the absence of an X-ray cycle and confirming the analogous results derived for $\epsilon$ Eridani. Finally, we establish an empirical relation between the cycle amplitude and the X-ray surface flux . From the absence of a coronal cycle in Kepler-63 we infer that stars with higher X-ray flux than Kepler-63 must host a significant fraction of higher-energetic flares than those necessary to model the corona of Kepler-63. Our study opens new ground for studies of the solar-stellar analogy and the joint exploration of resolved and unresolved variability in stellar X-ray lightcurves.

15 pages; 2 tables; 12 figures. Accepted for publication in A&A. Abstract shortened for the arXiv listing

References in corpus (4)

• X-ray Super-Flares From Pre-Main Sequence Stars: Flare Energetics And Frequency
• An X-ray activity cycle on the young solar-like star $\epsilon \mathrm{Eridani}$
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• SRG/eROSITA 3D mapping of the ISM using X-ray absorption spectroscopy
• The Sun as an X-ray star V.: A new method to retrieve coronal filling factors