Evolution of the surface rotation rate (top) and X-ray luminosity (bottom) of KIC 8006161. The yellow shaded areas indicate the region explored by the tracks across the evolution of the star, for 3.2 ≤ Ω_in/Ω_⊙ ≤ 18. The gray shaded areas show 20% (top) and global one order of magnitude (bottom) further variation at the limit of the yellow areas, to account for deviations from more likely rotational histories and fluctuation in the X-ray luminosity due to cyclic or stochastic magnetic activity. The red-dashed line in the top panel shows the evolution of the critical rotation rate, defined as the limit at which the centrifugal acceleration at the equator equals gravity. The blue markers show observational data. For comparison, evolutionary tracks for a 1 M_⊙ star are showed, with observational data from Table C.1.