[O   III]  versus X-ray luminosity for the SUNNS galaxies. In the left panel we use the  [O   III]  measurements from the Palomar survey, in the right panel we report the STIS data. The symbol colors are coded with the EW[O III] values measured in the relative spectrum: green is for objects with EW[O III]  >  2 Å, cyan for 1 Å  <  EW[O III] Å, and empty for EW[O III] 1 Å. We denote the sources that are detected in the radio with large purple pentagons. The dashed line reproduces the correlation between L _[O   III]  and L_X obtained by Panessa et al. (2006) for a sample of 13 nearby type I Seyfert galaxies; the dotted line marks the locus of constant ratio between line and X-ray luminosities normalized to the value measured by Heckman et al. (2005) in a sample of 20 type I AGNs with bright emission lines, i.e., log L_X/L _[O   III]  = 1.59. The median  [O III]  luminosities of these two samples are log L _[O   III]   ~  40.0 and  ~41.6, respectively.