Identification of Quasi-Periodic Oscillations of a black hole system

The inner regions of an accretion disk around a black hole produce X-rays which sometimes vary in time as Quasi-periodic Oscillations (QPOs), whose frequencies must correspond to some characteristic time-scale of the system associated with perhaps the inner truncated disk radius. These time-scales probe the strong gravitational field of the black hole. However, this identification has been difficult since one needs to simultaneously measure the inner radius (through broadband spectral studies) and the QPO frequency. AstroSat provides such a unique opportunity and we report on the variation of the QPO frequency with inner radius and accretion rate inferred by fitting relativistic spectral models. We find that while the QPO frequency depends on the inner radii with a large scatter, a much tighter correlation is obtained when both the inner radii and accretion rate of the disk are taken into account. We find that the frequency varies just as the dynamic frequency ( i.e. the inverse of the sound crossing time r/c s (r)) as predicted decades ago by the relativistic standard accretion disk theory for a black hole with a spin parameter of ∼0.9. We show that this identification has been possible due to the simultaneous broadband spectral coverage with temporal information as obtained from AstroSat.