2013MNRAS.436.3782D

We present the analysis of the X-ray variability and spectral timing properties of the ultraluminous X-ray source (ULX) NGC 5408 X-1, one of the most variable ULXs known so far. The variability properties are used as a diagnostic of the accretion state of the source and to derive estimates of the black hole (BH) mass. The observed high level of fast X-ray variability (fractional root-mean-square variability - rms - amplitude of ∼ 30 percent in the hard energy band), the hardening of the fractional rms spectrum and the properties of the quasi-periodic oscillation (QPO), all resemble those of a source in a hard-intermediate accretion state. We confirm the previous detection of a soft lag in the X-ray light curves of the source during 2006 and 2008 observations and find that the soft lag is still present in the more recent 2010/2011 observations. Using the entire available XMM-Newton data set (public as of 2012 February), we observe that the soft lag (of few seconds amplitude) is detected over a relatively large range of frequencies (ν ∼ 5-90mHz), which always includes the QPO frequencies. The soft lag displays energy-dependence, with the (absolute) amplitude increasing as a function of energy separation. We find close analogies with soft lags associated with type-C QPOs in BH binary systems (although an association to other types of QPOs cannot be completely excluded), as well as with reverberation lags observed in AGN. In both cases an intermediate-mass BH solution appears the most plausible.