Changing-look AGN: continuum vs. obscuration transformation

Spectral variability is a hallmark of AGN. Understanding the nature and origin of the spectral variability provides us with key insights into the accretion-outflow mechanism in AGN. We compare the remarkable cases of Seyfert-1 galaxies NGC 3516 and NGC 5548, where both have undergone major spectral transformations in recent years, however, the causes of their transformations are very different. We discuss the spectral diagnostics and modeling that help in distinguishing the origin of their transformations. In the case of NGC 3516, spectral changes are explained by the variability of the intrinsic continuum that result in the lowering of the ionization of the outflows in this AGN. This induces enhanced X-ray absorption by the outflows, mimicking presence of new obscuring gas. On the other hand, in the case NGC 5548, spectral changes are explained by the emergence of a new obscuring wind from the accretion disk. We show the striking X-ray and UV variability of this obscuring disk wind in NGC 5548, where its long-term decline are likely caused by an increasingly intermittent supply of outflowing winds from the accretion disk.