Logarithm of the hard‐band (0.52–2.02 keV) X‐ray luminosity, ﹩L^{h}_{\mathrm{X}\,}﹩, vs. the logarithm of the blue optical luminosity, L_B, for all of the galaxies in the sample. Group 1, 2, 3, and 4 galaxies are denoted by filled triangles, filled squares, open triangles, and open squares, respectively. Symbols with one‐sided vertical arrows are upper limits. The error bars or upper limits on the X‐ray luminosity are 1 σ. The six group 4 galaxies located at the centers of groups (Table 4) are shown as large open squares. The best‐fit power‐law relation is ﹩L^{h}_{\mathrm{X}\,}\propto L^{2.70\pm 0.20}_{B}﹩ if all galaxies are included (solid line) and ﹩L^{h}_{\mathrm{X}\,}\propto L^{2.48\pm 0.19}_{B}﹩ if the six group 4 galaxies located in the centers of groups are excluded (dotted line).