Observed 2–10 keV X-rays plotted against [O IV] line luminosity. Sy1 masers are represented by blue stars and Sy2 masers by red dots. Off-sample Sy2 masers are marked with black dots. The dotted lines mark fixed flux ratios of 1000; 100; 10; 1; 0.1 (from top to bottom). The error bar in the lower right corner is the average relative error of all [O IV] measurements.