A&A 652, A32 (2021) Astronomy https://doi.org/10.1051/0004-6361/202040192 & © J. Zhuleku et al. 2021 Astrophysics Stellar X-rays and magnetic activity in 3D MHD coronal models J. Zhuleku, J. Warnecke, and H. Peter Max Planck Institute for Solar System Research, 37077 Göttingen, Germany e-mail:
[email protected] Received 21 December 2020 / Accepted 28 May 2021 ABSTRACT Context. Observations suggest a power-law relation between the coronal emission in X-rays, LX, and the total (unsigned) magnetic flux at the stellar surface, Φ. The physics basis for this relation is poorly understood. Aims. We use three-dimensional (3D) magnetohydrodynamics (MHD) numerical models of the coronae above active regions, that is, strong concentrations of magnetic field, to investigate the LX versus Φ relation and illustrate this relation with an analytical model based on simple well-established scaling relations. Methods. In the 3D MHD model horizontal (convective) motions near the surface induce currents in the coronal magnetic field that are dissipated and heat the plasma. This self-consistently creates a corona with a temperature of 1 MK. We run a series of models that differ in terms of the (unsigned) magnetic flux at the surface by changing the (peak) magnetic field strength while keeping all other parameters fixed. Results. In the 3D MHD models we find that the energy input into the corona, characterized by either the Poynting flux or the total volumetric heating, scales roughly quadratically with the unsigned surface flux Φ. This is expected from heating through field-line 3:44 braiding. Our central result is the nonlinear scaling of the X-ray emission as LX Φ .