Possible time-variable iron-K$\alpha$ emission in the circumnuclear region of the Circinus galaxy

TL;DR

This study uses long-term Chandra observations to detect and analyze variable iron-Kα emission in the Circinus galaxy's circumnuclear region, suggesting X-ray echoes from AGN-illuminated clouds and potential feedback effects.

Contribution

First detection of time-variable iron-Kα emission on 10-100 pc scales in an external galaxy, indicating AGN X-ray echoes and feedback effects.

Findings

Detected iron-line emission 20-60 pc from the nucleus.

Observed possible time variability in the iron line flux.

Found that iron emission correlates with weak molecular gas regions.

Abstract

We present imaging and spatially resolved spectral analyses of eight Chandra data taken for the Circinus galaxy in $\approx$ 22 years to reveal neutral iron-K$\alpha$ emission on a circumnuclear scale ($\sim$ 10--100 pc) and search for time variability in the emission. By simulating and taking account of point-source emission from the active galactic nucleus (AGN), we detect iron-line emission $\sim$ 20--60 pc away from the nucleus, particularly in the eastern and western regions. In the two regions, possible time variability in the line flux was also detected. Our spectral analysis then finds that the observed equivalent widths can reach $\sim$ 2 keV and the slopes of underlying continua are rather inverted with $\Gamma < 0$. These are consistent with a scenario in which the iron emission originates from clouds illuminated by AGN X-rays; our result could provide the first extragalactic example of AGN X-ray echoes. In this scenario, we estimated the physical sizes of the illuminated clouds based on the timescale of variability to be less than 6 pc. Furthermore, we compared the iron emission distribution with the cold molecular distribution inferred by Atacama Large Millimeter/submillimeter Array (ALMA) observation of CO($J$=3--2), revealing that in the region of bright iron-line emission, the molecular emission seems to be weak. This might suggest that the AGN X-ray emission affects the chemical composition in the form of AGN feedback.