Using Faded Changing-look Quasars to Unveil the Spectral Energy Distribution Evolution of Low-luminosity Active Galactic Nuclei

The structure of accretion flows in low-luminosity active galactic nuclei (AGNs) at low Eddington ratios (∼10⁻²–10⁻³) are poorly understood, and can be probed using the spectral energy distributions (SEDs) of faded changing-look (CL) quasars. Previous results using single-epoch X-ray and rest-frame UV observations of samples of faded CL quasars suggest that their SED properties at low Eddington ratios display similarities to X-ray binaries fading from outburst. However, more robust tests demand multiepoch observations that can trace the temporal behavior of the SEDs of individual AGNs at low Eddington ratios. Here, we perform this test, by obtaining a second epoch of UV and X-ray observations of a sample of three faded CL quasars with bolometric Eddington ratios of ≲10⁻³, using a combination of contemporaneous Hubble Space Telescope UV imaging, Chandra X-ray observations, and optical spectroscopy. We find that all three CL quasars varied in luminosity, and their optical-to-X-ray spectral indices α _OX all individually display a negative (harder-when-brighter) correlation with Eddington ratio. This SED evolution is also often observed in X-ray binaries at low Eddington ratios, and adds to the growing evidence that AGN accretion flows behave analogously to X-ray binaries across all accretion states.