KIC 6048106: An Algol-type Eclipsing System with Long-term Magnetic Activity and Hybrid Pulsations. I. Binary modelling

Abstract: The A/F-type stars and pulsators ($\delta$ Scuti-$\gamma$ Dor) are in a critical regime where they experience the transition from radiative to convective transport of energy in their envelopes. Such stars can pulsate in both gravity and acoustic modes. Hence, the knowledge of their fundamental parameters along with their observed pulsation characteristics can help in improving the stellar models. When residing in a binary system, these pulsators provide more accurate and less model dependent stellar parameters than in the case of their single counterparts. We present a light curve model for the eclipsing system KIC 6048106 based on the \textit{Kepler} photometry and the code PHOEBE. We aim to obtain accurate physical parameters and tough constraints for the stellar modelling of this intermediate-mass hybrid pulsator. We performed a separate modelling of three light curve segments that show a distinct behaviour due to a difference in activity. We also analysed the \textit{Kepler} ETVs. KIC 6048106 is an Algol-type binary with F5-K5 components, a near-circular orbit and a 1.56-d period undergoing variations of the order of $\frac{\Delta P}{P}\simeq 3.60\times10^{-7}$ in $287\pm7$ days. The primary component is a main-sequence star with $M_{1}=1.55\pm0.11M_{\odot}$,~$R_{1}=1.57\pm0.12R_{\odot}$. The secondary is a much cooler subgiant with $M_{2}=0.33\pm0.07M_{\odot}$,~$R_{2}=1.77 \pm0.16R_{\odot}$. Many small near-polar spots are active on its surface. The second quadrature phase shows a brightness modulation on a time scale $290\pm7$ days, in good agreement with the ETV modulation. This study reveals a stable binary configuration along with clear evidence of a long-term activity of the secondary star.