  Quasi-Periodic Oscillations (QPOs) of the hot plasma spots or clumps orbiting
an accreting black hole contain information on the black hole mass and spin.
The promising observational signatures for the measurement of black hole mass
and spin are the latitudinal oscillation frequency of the bright spots in the
accretion flow and the frequency of black hole event horizon rotation. Both of
these frequencies are independent of the accretion model and defined completely
by the properties of the black hole gravitational field. Interpretation of the
known QPO data by dint of a signal modulation from the hot spots in the
accreting plasma reveals the Kerr metric rotation parameter, $a=0.65\pm0.05$,
and mass, $M=(4.2\pm0.2)10^6M_\odot$, of the supermassive black hole in the
Galactic center. At the same time, the observed 11.5 min QPO period is
identified with a period of the black hole event horizon rotation, and,
respectively, the 19 min period is identified with a latitudinal oscillation
period of hot spots in the accretion flow. The described approach is applicable
to black holes with a low accretion rate, when accreting plasma is transparent
up to the event horizon region.
