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We report on the analysis of hard-state power spectral density function (PSD)of GX 339-4 down to the soft X-ray band, where the disc significantlycontributes to the total emission. At any luminosity probed, the disc in thehard state is intrinsically more variable than in the soft state. However, thefast decrease of disc variability as a function of luminosity, combined withthe increase of disc intensity, causes a net drop of fractional variability athigh luminosities and low energies, which reminds the well-known behaviour ofdisc-dominated energy bands in the soft state. The peak-frequency of thehigh-frequency Lorentzian (likely corresponding to the high-frequency breakseen in active galactic nuclei, AGN) scales with luminosity, but we do not findevidence for a linear scaling. In addition, we observe that this characteristicfrequency is energy-dependent. We find that the normalization of the PSD at thepeak of the high-frequency Lorentzian decreases with luminosity at allenergies, though in the soft band this trend is steeper. Together with thefrequency shift, this yields quasi-constant high frequency (5-20 Hz) fractionalrms at high energies, with less than 10 percent scatter. This reinforcesprevious claims suggesting that the high frequency PSD solely scales with BHmass. On the other hand, this constancy breaks down in the soft band (where thescatter increases to ~30 percent). This is a consequence of the additionalcontribution from the disc component, and resembles the behaviour of opticalvariability in AGN.

The evolution of the disc variability along the hard state of the black hole transient GX 339-4