The expected GRE features for a BH mass of 5 × 10⁶ M_⊙ for the highly relativistic case. The black solid line indicates the BPL model with a high-frequency slope of −2, and a bend frequency of 8 × 10⁻⁵ Hz. The red dotted line shows the expected ‘highly relativistic’ BPLGRE model PSD (in the case of a Kerr BH and a point-like X-ray source located at a height of h = 3.6 r_g). The blue dashed lines indicate the best-fitting BPL model to the BPLGRE model PSD, when the high-frequency slope is fixed to −2. The attached plot at the bottom shows the ratio of the BPLGRE model PSD over the respective best-fitting BPL model (red dotted and blue dashed lines in the upper panel, respectively).