Probing the disk-corona systems and broad line regions of changing-look quasars with X-ray and optical observations

TL;DR

This study investigates the disk-corona systems and broad line regions of changing-look quasars through multi-epoch X-ray and optical observations, revealing links between spectral changes and Eddington ratios.

Contribution

It provides new observational evidence connecting spectral type changes in quasars to Eddington ratio thresholds, supporting the disk-wind model for broad emission line formation.

Findings

Changing-look quasars cross the 1% Eddington ratio boundary during spectral changes.

Spectral indices below 1% Eddington ratio show similarities to X-ray binary behavior.

Eddington ratios before and after spectral type changes support the disk-wind model.

Abstract

"Changing-look" quasars are a new class of highly variable active galactic nuclei that have changed their spectral type over surprisingly short timescales of just a few years. The origin of this phenomenon is debated, but is likely to reflect some change in the accretion flow. To investigate the disk-corona systems in these objects, we measure optical/UV-X-ray spectral indices ($\alpha_{\rm OX}$) and Eddington ratios ($\lambda_{\rm Edd}$) of ten previously-discovered changing-look quasars at two or more epochs. By comparing these data with simulated results based on the behavior of X-ray binaries, we find possible similarities in spectral indices below 1% Eddington ratio. We further investigate the Eddington ratios of changing-look quasars before and after their spectral type changes, and find that changing-look quasars cross the 1% Eddington ratio boundary when their broad emission lines disappear/emerge. This is consistent with the disk-wind model as the origin of broad emission lines.