Dehghanian, Maryam ; Ferland, Gary ; Kriss, Gerard ; Peterson, Bradley ; Guzman, Francisco ; Chatzikos, Marios ; van Hoof, Peter A. M.

published in American Astronomical Society Meeting Abstracts, 233, pp. 243.11 (2019)

Abstract: The absorption and emission lines produced in a photoionized cloud correlate with changes in the ionizing radiation field. This correlation, and the delay between changes in lines and the continuum, are used in reverberation mapping to measure the size of the source and mass of its central black hole in AGNs. The Space Telescope and Optical Reverberation Mapping Project (STORM), the largest-ever AGN monitoring campaign, surprisingly discovered that this correlation was violated in NGC 5548. The soft X-ray part of the SED was dramatically extinguished by an obscurer, a phenomenon seen in this object in 2013 by Kaastra et al. During part of the time that this obscurer was present, the absorption and emission lines did not respond to variations of the continuum. Here we model the decorrelation of the absorption lines from the continuum in terms of a varying obscurer covering factor, and identify the physics which makes this possible. We identify a cycle in which the soft X-ray portion of the SED varies, causing changes in the ionization of helium. The ionizing radiation produced in its recombination governs the ionization of the species observed with HST. Photoionization models reproduce the sense of the HST observations. The obscurer is likely to be part of the broad-line region which happens to cover our sight line to the central object. This shows the importance of cloud shadowing in understanding the physics of the emission-line clouds.