Multi-wavelength properties of changing-state active galactic nuclei: I. the evolution of soft excess and X-ray continuum

TL;DR

This study analyzes over 1000 X-ray observations of five changing-state active galactic nuclei to understand the evolution of their spectral components, revealing correlations between soft excess, continuum emissions, and accretion rates, and suggesting a change in accretion flow geometry at low Eddington ratios.

Contribution

It provides the first comprehensive analysis of the broadband X-ray spectral evolution of CSAGNs across a wide range of Eddington ratios, highlighting the origin of soft excess and accretion flow changes.

Findings

Strong positive correlation between soft excess and primary continuum.

Soft excess is not dominated by blurred ionized reflection.

Soft excess vanishes below log λ_Edd ≈ -2.5.

Abstract

Changing-state active galactic nuclei (CSAGNs) exhibit rapid variability, with mass accretion rates that can change by several orders of magnitude in a few years. This provides us with a unique opportunity to study the evolution of the inner accretion flow almost in real time. Here, we used over 1000 observations to study the broadband X-ray spectra of a sample of five CSAGNs, spanning three orders of magnitude in Eddington ratio ($\lambda_{\rm Edd}$), using phenomenological models to trace the evolution of key spectral components. We derive several fundamental parameters, such as the photon index, soft excess strength, reflection strength, and luminosities of the soft excess and primary continuum. We find that the soft excess and primary continuum emissions show a very strong positive correlation ($p \ll 10^{-10}$), suggesting a common physical origin. The soft excess strength does not show any dependency on the reflection parameter, suggesting that in these objects the soft excess is not dominated by a blurred ionized reflection process. On the other hand, the strength of the soft excess is found to be strongly positively correlated with the Eddington ratio ($p \ll 10^{-10}$), and we find that the soft excess vanishes below $\log \lambda_{\rm Edd} \sim -2.5$. Moreover, we find a clear `V'-shaped relation for $\Gamma-\lambda_{\rm Edd}$, with a break at $\log \lambda_{\rm Edd} = -2.47 \pm 0.09$. Our findings indicate a change in the geometry of the inner accretion flow at low Eddington ratios, and that the soft excess is primarily produced via warm Comptonization.