Universal scaling of the 3:2 twin-peak quasi-periodic oscillation frequencies with black hole mass and spin revisited

We discuss further observational support of an idea formulated a decade ago by Abramowicz, Klu{'z}niak, McClintock and Remillard. They demonstrated that the 3:2 pairs of frequencies of the twin-peak black hole (BH) high-frequency quasi-periodic oscillations (QPOs) scale inversely with the BH masses and that the scaling covers the entire range from stellar to supermassive BHs. For this reason, they believed that the QPOs may be used for accurate measurements of masses and spins of BHs.