Response of the Fe Kalpha line emission to the X-ray continuum variability in the changing-look active galactic nucleus NGC 1566
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 Kalpha 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 Kalpha 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 Kalpha line. In addition, we find the response of narrow Fe Kalpha 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 Kalpha variation rate (the ratio of Fe Kalpha 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 Kalpha line for the latter. The strength of variable broad Fe Kalpha 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.