The NuSTAR view of five changing-look active galactic nuclei

Bing Lyu, Zhen Yan, Xue-bing Wu, Qingwen Wu, Wenfei Yu, Hao Liu

First listed 2025-01-16 | Last updated 2025-01-16

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 ($Γ$) 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.

Short digest

NuSTAR multi-epoch X-ray spectra of five CLAGNs (ESO 362-G18, NGC 1365, NGC 4151, NGC 5548, NGC 7582) are fit with phenomenological and clumpy-torus models to track Γ, L2–10 keV/LEdd, and NH,los. The authors find a positive Γ–(L2–10 keV/LEdd) correlation and, for four objects, a strong anti-correlation between NH,los and L2–10 keV/LEdd, with ESO 362-G18 standing out as an exception. They argue this links changing-obscuration to accretion-driven evolution: a disc wind that strengthens with accretion rate pushes line-of-sight clouds outward, lowering NH at moderate inclinations. The trends persist across model choices, tying CLAGN absorption changes to the same engine that drives changing-state behavior.

Key figures to inspect

• Figure 1: Inspect the Γ–L2–10 keV/LEdd and NH,los–L2–10 keV/LEdd relations from the pexrav fits; note the best-fit line and 3σ band, and verify ESO 362-G18 as the clear outlier in NH–Edd ratio. Use the symbol mapping to follow each source’s trajectory across epochs and compare against grey literature points.
• Figure 2: Compare the same correlations using two clumpy-torus models to see if the Γ–Edd and NH–Edd trends survive once reprocessing/geometry are treated more physically; check how each source shifts relative to the pexrav results.
• Figure 3: Re-examine the clumpy-torus correlations when bolometric corrections are applied (Netzer 2019); assess whether slopes or scatter in Γ–Edd and NH–Edd change, and whether the ESO 362-G18 deviation persists.
• Figure 4: Look at the distribution of correlation slopes for NH,los vs. L2–10 keV/LEdd and the corresponding null-hypothesis probabilities; this quantifies which sources and models yield statistically robust anti-correlations.