2015A&A...578A.129J

We study the X-ray emission of low-mass main-sequence stars to derive a reliable general scaling law between coronal temperature and the level of X-ray activity. We collect ROSAT measurements of hardness ratios and X-ray luminosities for a large sample of stars to derive which stellar X-ray emission parameter is most closely correlated with coronal temperature. We calculate average coronal temperatures for a sample of 24 low-mass main-sequence stars with measured emission measure distributions (EMDs) collected from the literature. These EMDs are based on high-resolution X-ray spectra measured by XMM-Newton and Chandra. We confirm that there is one universal scaling relation between coronal average temperature and surface X-ray flux, F_X, that applies to all low-mass main-sequence stars. We find that coronal temperature is related to F_X by T_cor=0.11F_X^0.26, where T_cor is in MK and F_X is in erg/s/cm2.