Optical depth vs electron coronal temperature in the case of a spherical corona for GRS 1734-292, NGC 5506, and MCG-05-23-16 (blue circles) and the values obtained with our reanalysis (green triangle); see Appendix A. We superimposed the fit (red line) and the error on the fitting relation (red shaded region) obtained with all the literature values for all the sources, as in the lower panel of Fig. 6.