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

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 similar to 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 similar to 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. (c) 2022 Elsevier B.V. All rights reserved.