Insight into "Changing-look" AGN Mrk 1018 from the Fe Kα Line: The Reprocessing Gas Has Yet to Fully Respond to the Fading of the AGN

Mrk 1018 is a "changing-look" active galactic nucleus (AGN) whose optical spectrum transitioned from a Type 1.9 to a Type 1 between 1979 and 1984, and then back to a Type 1.9 in 2015. This latest transition was accompanied by a decrease in X-ray flux. We analyze the Chandra spectra from 2010 and 2016 and NuSTAR spectra from 2016, with a careful treatment of pileup in the Chandra spectrum from 2010, and self-consistently model absorption, reflection, and Fe Kα line emission in the X-ray spectra from 2016. We demonstrate that while the 2–10 keV X-ray flux decreased by an order of magnitude (1.46${}_{-0.13}^{+0.10}\times {10}^{-11}$–${1.31}_{-0.04}^{+0.09}\times {10}^{-12}$ erg s⁻¹ cm⁻²), the Fe Kα equivalent width (EW) increased from ${0.18}_{-0.12}^{+0.17}$ to ${0.61}_{-0.25}^{+0.27}$ keV due to a depressed AGN continuum. We jointly fit the Chandra and NuSTAR spectra from 2016 using the physically motivated MYTorus model, and find that the torus orientation is consistent with a face-on geometry and that lines of sight intersecting the torus are ruled out. While we measure no line-of-sight absorption, we do measure a column density of $N_H={5.38}_{-4.0}^{+14}\times {10}^{22}$ cm⁻² for gas out of the line of sight that reprocesses the X-ray emission. We find a high relative normalization between the Compton-scattered emission and transmitted continuum, which is indicative of time lags between the primary X-ray source and reprocessing gas. We predict that the Fe Kα line will respond to the decrease in AGN flux, which would manifest as a decrease in the Fe Kα EW.