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Context. Obscuration events caused by outflowing clumps or streams of highcolumn density, low ionisation gas, heavily absorbing the X-ray continuum, havebeen witnessed in a number of Seyfert galaxies. Aims. We report on the X-rayspectral-timing analysis of the December 2016 obscuration event in NGC 3783,aimed at probing variability of the X-ray obscurer on the shortest possibletimescales. The main goals of this study are to obtain independent constraintson the density, and ultimately on the distance of the obscuring gas, as well asto characterise the impact of variable obscuration on the observed X-rayspectral-timing characteristics of Seyfert galaxies. Methods. We carried out acomparative analysis of NGC 3783 during unobscured (using archival 2000-2001XMM-Newton data) and obscured states (using XMM-Newton and NuSTAR data from the2016 observational campaign). The timescales analysed range between ten hoursand about one hour. This study was then generalized to discuss the signaturesof variable obscuration in the X-ray spectral-timing characteristics of Seyfertgalaxies as a function of the physical properties of the obscuring gas.Results. The X-ray obscurer in NGC 3783 is found to vary on timescales betweenabout one hour to ten hours. This variability is incoherent with the variationsof the X-ray continuum. A fast response (on timescales shorter than about 1.5ks) of the ionisation state of the obscuring gas to the short timescalevariability of the primary X-ray continuum provides a satisfactoryinterpretation of all the observed X-ray spectral-timing properties. This studyenabled us to put independent constraints on the density and location of theobscuring gas. We found the gas to have a density of $n_{e}> 7.1 \times 10^7\rm{cm^{-3}}$, consistent with being part of the broad line region.

Incoherent fast variability of X-ray obscurers. The case of NGC 3783