The QPO phenomena and their time lags in five ULXs

We investigated the power density spectrum (PDS) and time lag of 5 Ultraluminous X-ray Sources (ULXs) observed by XMM-Newton, which show quasi-periodic oscillations (QPOs). The motivation underneath is that, the correlations between the frequency of QPO and the BH mass, and between the time lag and BH mass provide us a chance of probing in time domain the mas of the compact object harbored in ULXs. As a result we found from these five ULXs for both correlations, which suggests that the compact objects are black holes with intermediate sizes if puts these relations into a context established by stellar black holes and supermassive black holes.