Normalised excess variance versus log(L_{2−10   keV}) for ULXs (black squares, and open square for the M 82 X−1 data) and AGN (open triangles). IMBHs reported by Miniutti et al. (2009) are included as black stars and POX 52 (reported here) as an open circle. Top panel: the solid line indicates the best fit to the ULX data, and the dashed line indicates the same line shifted by +4 along the x-axis. Middle panel: the lines indicate the case A model “variability-luminosity” relation for ṁ_Edd = 0.03 (dotted line), 0.1 (dashed line), and 0.3 (continuous line). The short solid lines between the model curves indicate black hole masses of 5 × 10⁶,   5 × 10⁷, and 5 × 10⁸   M_⊙, from left to right. Bottom panel: the lines indicate the case B model “variability-luminosity” relation, for the same accretion rates. As above, the short solid lines within the model curves indicate black hole masses of 10³,   10⁴,   5 × 10⁵,   5 × 10⁶,   5 × 10⁷, and 5 × 10⁸   M_⊙from left to right.