A XRISM Study of Highly Ionized Iron Emission Lines from the Low-Eddington-ratio AGN in NGC 7213

TL;DR

This study analyzes XRISM and NuSTAR data of NGC 7213 to understand the origin of highly ionized iron lines, exploring photoionization and collisional ionization processes, and compares it with M 81* to infer density trends.

Contribution

It provides the first detailed analysis of iron emission lines in NGC 7213 using XRISM data, assessing ionization mechanisms and their relation to Eddington ratio.

Findings

Fe XXV and Fe XXVI lines may have different velocity widths.

Current data cannot definitively distinguish between photoionization and collisional ionization.

Gas density appears to decrease with lower Eddington ratio.

Abstract

We present an analysis of XRISM and NuSTAR data obtained for the nearby low-Eddington active galactic nucleus NGC 7213. Our goal is to examine whether its He-like and H-like iron emission lines can be reproduced by photoionization or collisional ionization processes. Using the broad-band energy coverage of our data (2-60 keV), we first constrained the continuum shape. Then, we focused on the iron-K band in the Resolve spectrum. Gaussian fits to Fe XXV He$\alpha$ and Fe XXVI Ly$\alpha$ lines suggest that they may have different velocity widths: $v_\sigma=790^{+370}_{-240}$ km s$^{-1}$ for Fe XXV and $v_\sigma=2610^{+1700}_{-1580}$ km s$^{-1}$ for Fe XXVI. In this case, the He$\alpha$ resonance line (w) and forbidden line (z) have similar intensities of $\approx0.5$-$0.6\times10^{-5}$ ph s$^{-1}$ cm$^{-2}$, while the intercombination lines (x+y) are not significantly detected with upper limits of $\lesssim 0.2\times10^{-5}$ ph s$^{-1}$ cm$^{-2}$. Motivated by the possible difference in the line widths, we tested one- and two-zone photoionized and collisionally ionized models. Our results show that the additional ionized component is not significantly required, and the current data cannot uniquely determine whether photoionization or collisional ionization dominates. Moreover, if the Fe XXV He$\alpha$ complex implies that the weak x+y lines are suppressed relative to the w and z lines, such a structure is difficult to reproduce with either ionization model adopted. Finally, by comparing NGC 7213 with M 81$^\ast$, accreting at a much lower Eddington ratio of $\lambda_{\rm Edd}\sim 10^{-5}$, we found a decrease in the density of the gas responsible for highly ionized iron emission, which may imply that the density decreases with decreasing $\lambda_{\rm Edd}$.