Comparison between the HE γ-ray photon indices for the LAT-detected sources in the sample and their optical luminosities. The color-coding denotes the Pearson’s correlation coefficient of the optical CM relation, C(B − R,R), as depicted in the stripe below the diagram. Different symbols here are used to denote three types of BL Lac objects included in the sample: HBLs (circles), LBLs (triangles), and LBLs (stars). The error bars for LR are evaluated as described in Sect. 2.4. The error bars for the Fermi-LAT spectral indices are taken from 2FGL.
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