Response of the Fe K_alpha line emission to the X-ray continuum variability in the changing-look active galactic nucleus NGC 1566

TL;DR

This study analyzes X-ray observations of the changing-look AGN NGC 1566, revealing that the broad Fe K_alpha line responds to continuum variability and suggesting a close link between accretion processes and spectral features during outbursts.

Contribution

It provides the first evidence of the broad Fe K_alpha line responding to X-ray continuum changes in NGC 1566, supporting disk reflection models in changing-look AGNs.

Findings

Broad Fe K_alpha line flux increases with continuum during outburst

Narrow Fe K_alpha line responds within four months, indicating a <0.1 pc emitting region

Correlation between Fe K_alpha line strength, soft X-ray excess, and accretion rate

Abstract

NGC 1566 is a changing look AGN known to exhibit recurrent X-ray outbursts with each lasting for several years. The most recent X-ray outburst is observed on 2018, with a substantial increase of 2--10 keV flux by a factor of ~24 than the historical minimum. We re-analyze the XMM-Newton and NuSTAR observations covering the pre-outburst, outburst and post-outburst epochs, and confirm the discovery of the broad feature in the ~5--7 keV band during the period of outburst that could be interpreted as a relativistic Fe K_alpha emission line. Our analysis suggests that its flux has increased in tandem with the 2--10 keV continuum, making it the second changing look AGN in which the broad Fe K_alpha line responds to the X-ray continuum variability. This behavior strongly supports the idea that X-rays originates in a corona above the accretion disk, and disk reflection produces the relativistic Fe K_alpha line. In addition, we find the response of narrow Fe K_alpha emission line to the changes in the X-ray continuum on a time-scale as short as four months, allowing to put the location of line-emitting region at <0.1 pc, comparable to the size of optical BLR. By comparing to the changing look AGN NGC 2992, the Fe K_alpha variation rate (the ratio of Fe K_alpha variation to luminosity variation) in NGC 1566 appears greater, which could be possibly explained by larger amount of gas or Fe abundance responsible for producing the Fe K_alpha line for the latter. The strength of variable broad Fe K_alpha line as well as the soft X-ray excess emission appears to be correlated with the accretion rate, which could be explained as due to the state transition associated with the changing-look phenomenon.