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We show that the Fe (VII-XII) M-shell unresolved transition array (UTA) inthe NGC 3783 900 ks Chandra HETGS observation clearly changes in opacity in atimescale of 31 days responding to a factor of 2 change in the ionizingcontinuum. The opacity variation is observed at a level >10 sigma. There isalso evidence for variability in the O VI K edge (at 3 sigma level). Theobserved changes are consistent with the gas producing these absorptionfeatures (the low ionization component) being close to photoionizationequilibrium. The gas responsible for the Fe (XVII-XXII) L-shell absorption (thehigh ionization component), does not seem to be responding as expected inphotoionization equilibrium. The observed change in opacity for the UTA impliesa 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 ofionization structures is ruled out, as in such scenario, no opacity variationsare expected. Rather, the structure of the absorber is likely composed byheavily clumped gas.Comment: 14 pages, 2 figures, 1 table. Accepted for publication in Ap

Opacity Variations in the Ionized Absorption in NGC 3783: A Compact Absorber