Bifurcation timescales in power spectra of black hole binaries and ultraluminous X-ray sources

For black hole binaries (BHBs) and active galactic nuclei (AGNs), bifurcation timescales (BTs) Δt _b exist, below which time-domain power is significantly higher than the corresponding Fourier power. Quasi-periodic oscillations (QPOs) are removed from the Fourier spectra of BHBs. A relationship between BT, black hole mass and bolometric luminosity is derived. Strong anti-correlation between BT and luminosity of Cyg X-1 is found. After removing the QPOs, BTs are also obtained for two ultraluminous X-ray sources (ULXs), M82 X-1 and NGC5408 X-1. The results support that they harbor intermediate mass black holes (IMBHs).