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

TL;DR

This study uses multi-epoch UV and X-ray observations of faded changing-look quasars to investigate the spectral energy distribution evolution of low-luminosity active galactic nuclei, revealing behaviors similar to X-ray binaries at low Eddington ratios.

Contribution

It provides the first multi-epoch analysis of individual low-Eddington ratio AGN SEDs, demonstrating their spectral evolution and analogy to X-ray binary states.

Findings

All three quasars varied in luminosity.

Optical-to-X-ray spectral index alpha_OX correlates negatively with Eddington ratio.

SED evolution resembles that of X-ray binaries at low accretion rates.

Abstract

The structure of accretion flows in low-luminosity active galactic nuclei (AGN) at low Eddington ratios (~10^-2 to 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 multi-epoch observations that can trace the temporal behavior of the SEDs of individual AGN 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 HST 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 alpha_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.