GJ 357 compared to the activity–rotation–age relation of M dwarfs from Magaudda et al. (2020). Left: X-ray luminosity vs. rotation period; for GJ 357 the L_x measurement is derived from recent XMM-Newton data analysed in this paper and the P_rot value is from Luque et al. (2019). Right: X-ray luminosity vs. age reconstructed based on the observed L_x − P_rot relation (left panel) and the angular momentum evolution models of Matt et al. (2015). In addition, the empirical scaling law from Penz & Micela (2008) is shown. Two M stars with known ages, Barnard’s star (GJ 699) and Kapteyn’s star (GJ 191), are added to the plots for reference.