Response of the Fe K_alpha 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 ~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.
