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

We show that the Fe VII-Fe XII M-shell unresolved transition array (UTA) in the Chandra HETGS observation of NGC 3783 (900 ks) clearly changes in opacity on 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 σ. There is also evidence for variability in the O VI K edge (at ~3 σ). The observed changes are consistent with the gas producing these absorption features (i.e., the low-ionization component) being close to photoionization equilibrium. The gas responsible for the Fe XVII-Fe XXII L-shell absorption (i.e., 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 >1 × 104 cm-3, thus locating 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 of heavily clumped gas.