Insight Into "Changing-Look" AGN Mrk 1018 from the Fe K$\alpha$ Line: The Reprocessing Gas Has Yet to Fully Respond to the Fading of the AGN

Abstract

Mrk 1018 is a "changing-look" AGN whose optical spectrum transitioned from a Type 1.9 to Type 1 between 1979 and 1984 and 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, carefully treating pile-up in the Chandra spectrum from 2010 and self-consistently modeling absorption, reflection, and Fe K$\alpha$ 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$^{+0.10}_{-0.13} \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ to 1.31$^{+0.09}_{-0.04} \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$), the Fe K$\alpha$ equivalent width (EW) increased from 0.18$^{+0.17}_{-0.12}$ to 0.61$^{+0.27}_{-0.25}$ keV, due to a depressed AGN continuum. We jointly fit the Chandra and NuSTAR spectra from 2016 using the physically-motivated MYTorus model, finding that the torus orientation is consistent with a face-on geometry, and lines of sight intersecting the torus are ruled out. While we measure no line-of-sight absorption, we measure a column density of $N_{\rm H}$ = 5.38$^{+14}_{-4.0} \times 10^{22}$ cm$^{-2}$ for gas out of the line-of-sight which reprocesses the X-ray emission. We find a high relative normalization between the Compton scattered emission and transmitted continuum, indicative of time lags between the primary X-ray source and reprocessing gas. We predict that the Fe K$\alpha$ line will respond to the decrease in AGN flux, which would manifest as a decrease in the Fe K$\alpha$ EW.