X-ray photon flux vs. the Spearman’s correlation coefficient. Different symbols are used to mark stars in three different ranges of T_eff, as shown in the label. The vertical lines delimit the locus of the anti-correlated and correlated groups. The horizontal dotted lines instead mark the median values of photon flux in the three groups. not correlated stars have on average larger X-ray luminosity than the stars in the other two samples.