The NuSTAR view of five changing-look active galactic nuclei

TL;DR

This study analyzes X-ray spectra of five changing-look active galactic nuclei using NuSTAR data, revealing correlations between X-ray properties and supporting the disc wind scenario for their spectral changes.

Contribution

It provides new insights into the physical mechanisms behind changing-look AGNs by modeling their X-ray spectra and exploring the connection between accretion disc evolution and obscuration changes.

Findings

Positive correlation between photon index and Eddington-scaled luminosity.

Negative correlation between line-of-sight column density and luminosity.

Support for the disc wind scenario in changing-look AGNs.

Abstract

Changing-look active galactic nuclei (CLAGNs) are known to change their spectral type between 1 and 2 (changing-state) or change their absorption between Compton-thick and Compton-thin (changing-obscuration) on timescales of years or less. The physical mechanism and possible connection between the two types of CLAGNs are still unclear. We explore the evolution of the broadband X-ray spectra from Nuclear Spectroscopic Telescope Array (\nustar\,) and column density in five CLAGNs with moderate inclination viewing angles, which have shown significant variations of both optical types and X-ray absorption. Based on a phenomenological and two clumpy torus models, we find that the X-ray photon index ($\Gamma$) and the Eddington-scaled X-ray $2-10$ keV luminosity ($L_{\rm X}/L_{\rm Edd}$) are positively correlated for the five sources, which are similar to other bright AGNs and optical CLAGNs at type 1 phase. We find a significant negative correlation between log$N_\mathrm{H,los}$ and log$L_{\rm X}/L_{\rm Edd}$ except for ESO 362-G18. Similar to changing-state AGNs, changing-obscuration AGNs may be also triggered by the evolution of the accretion disc. Our results support the disc wind scenario, where the disc wind proportional to the accretion rate and formed at moderate inclination angles would push the obscuration material further away and decrease the column density from the line of sight observed in the changing-look AGNs.