Flat Power Spectra of Ultraluminous X-ray Sources: Evidence of Super-Eddington Accretion

We analyze a stochastic X-ray variability of ultraluminous X-ray sources (ULXs) using a sample of 12 objects. Recently, we have found that the power spectrum of SS 433 can be fitted by the broken power law with a flat spectrum between 10^-5 and 10^-3 Hz. Similar flat power spectra are shown in ultraluminous X-ray sources NGC 5408 X-1, NGC 6946 X-1, and M82 X-1. These objects also have quasi-periodic oscillations (QPO) near 0.01 Hz. We suppose that such a type of variability may be related to the structure of supercritical disk and they may represent an evidence of supercritical accretion in these sources.