  We show that the Fe (VII-XII) M-shell unresolved transition array (UTA) in
the NGC 3783 900 ks Chandra HETGS observation clearly changes in opacity in a
timescale of 31 days responding to a factor of 2 change in the ionizing
continuum. The opacity variation is observed at a level >10 sigma. There is
also evidence for variability in the O VI K edge (at 3 sigma level). The
observed changes are consistent with the gas producing these absorption
features (the low ionization component) being close to photoionization
equilibrium. The gas responsible for the Fe (XVII-XXII) L-shell absorption (the
high ionization component), does not seem to be responding as expected in
photoionization equilibrium. The observed change in opacity for the UTA implies
a density >1E4 cm-3, and so locates the gas within 6 pc of the X-ray source.
The scenario in which the gas is composed of a continuous radial range of
ionization structures is ruled out, as in such scenario, no opacity variations
are expected. Rather, the structure of the absorber is likely composed by
heavily clumped gas.
