Observed X-ray to optical ratio as a function of galaxy stellar mass. Squares indicate X-ray detections and arrows 3a- upper limits, color-coded by the waveband used for variability selection. The luminosity from the stacked images of non-detected galaxies are shown with red stars (and their uncertainties with shaded contours, see the text in Sect. 6). The black contour indicate the predicted X/O from normal galaxies (Lehmer et al. 2019), the dotted black line its extrapolation and the dashed black line a tentative correction for the low-mass end (Gilfanov et al. 2004; Lehmer et al. 2019, and refer to Sect. 4.2 in this work). Red lines show the predicted X/O for AGN in their radiatively efficient phase (solid red line; Arcodia et al. 2019), compared to inefficient ones at ~10⁻³ or ~10⁻⁴ L_edd (dashed and dotted red lines; Merloni 2016; Ruan et al. 2019).