Black hole mass as a function of the semi-major axis of the hotspot’s orbit. The orbital period is fixed to the observed one, P_s = 75.4 days. Peng et al. (2006) estimates of the black hole mass imply that the emission region of the disk at the origin of the periodic signal is located at around 120r_g from the central black hole. The vertical blue line indicates the ISCO for a Schwarzschild black hole.