  Outflows in active galactic nuclei (AGN) are considered a promising candidate
for driving AGN feedback at large scales. However, without information on the
density of these outflows, we cannot determine how much kinetic power they are
imparting to the surrounding medium. Monitoring the response of the ionisation
state of the absorbing outflows to changes in the ionising continuum provides
the recombination timescale of the outflow, which is a function of the electron
density. We have developed a new self-consistent time-dependent photoionisation
model, TPHO, enabling the measurement of the plasma density through
time-resolved X-ray spectroscopy. The algorithm solves the full-time-dependent
energy and ionisation balance equations in a self-consistent fashion for all
the ionic species. The model can therefore reproduce the time-dependent
absorption spectrum of ionized outflows responding to changes in the ionizing
radiation of the AGN. We find that when the ionised gas is in a non-equilibrium
state its transmitted spectra are not accurately reproduced by standard
photoionisation models. Our simulations with the current X-ray grating
observations show that the spectral features identified as a
multiple-components warm absorber, might be in fact features of a time-changing
warm absorber and not distinctive components. The TPHO model facilitates
accurate photoionisation modelling in the presence of a variable ionising
source, thus providing constraints on the density and in turn the location of
the AGN outflows. Ascertaining these two parameters will provide important
insight into the role and impact of ionised outflows in AGN feedback.
