X-ray variability of HD 189733 across eight years of XMM-Newton observations

Abstract: The characterization of exoplanets, their formation, evolution, and chemical changes is tightly linked to our knowledge of their host stars. In particular, stellar X-rays and UV emission have a strong impact on the dynamical and chemical evolution of planetary atmospheres. We analyzed 25 XMM-Newton observations encompassing about eight years and totaling about 958 ks in order to study the X-ray emission of HD 189733 A. We find that the corona of HD 189733 A has an average temperature of 0.4 keV and it is only during flares that the mean temperature increases to 0.9 keV. Apart from the flares, there is no significant change in the flux and hardness of the coronal emission on a timescale of several months to years. Thus, we conclude that there is no detectable activity cycle on such timescales. We identified the flares and built their energy distribution. The number of flares observed around the phases of the planetary eclipses is not statistically different from the number of flares during transit phases. However, we do find a hint of a difference in the flare-energy distributions, as the flares observed around the planetary eclipses tend to be more energetic than the flares observed around the primary transits of the planet. We modeled the distribution of the number of flares per day with a power law, showing that it is steeper than the one observed in the Sun and in other Main Sequence stars. The steepness hints at a significant fraction of undetected micro-flares. Altogether, the plasma temperatures below 1 keV observed during the flares, along with the slightly larger fraction of energetic flares seen at the secondary transits highlight the peculiarity of the corona of HD 189733 A and points to star-planet interaction as the plausible origin of part of its X-ray emission. However, more observational and modeling efforts are required to confirm or disprove this scenario.