  Ultrafast outflows (UFOs) are the most extreme winds launched by active
galactic nuclei (AGN) due to their mildly-relativistic speeds (~0.1-0.3c) and
are thought to significantly contribute to galactic evolution via AGN feedback.
Their nature and launching mechanism are however not well understood. Recently,
we have discovered the presence of a variable UFO in the narrow-line Seyfert 1
IRAS 13224-3809. The UFO varies in response to the brightness of the source. In
this work we perform flux-resolved X-ray spectroscopy to study the variability
of the UFO and found that the ionisation parameter is correlated with the
luminosity. In the brightest states the gas is almost completely ionised by the
powerful radiation field and the UFO is hardly detected. This agrees with our
recent results obtained with principal component analysis. We might have found
the tip of the iceberg: the high ionisation of the outflowing gas may explain
why it is commonly difficult to detect UFOs in AGN and possibly suggest that we
may underestimate their actual feedback. We have also found a tentative
correlation between the outflow velocity and the luminosity, which is expected
from theoretical predictions of radiation-pressure driven winds. This trend is
rather marginal due to the Fe XXV-XXVI degeneracy. Further work is needed to
break such degeneracy through time-resolved spectroscopy.
