A XRISM view of the iron line complex in NGC 1068: Rethinking the prototypical Compton-thick AGN

TL;DR

This study uses XRISM observations of NGC 1068 to analyze iron emission lines, revealing a complex, stratified environment with distinct neutral and ionized regions, and suggests a bipolar outflow as a feedback mechanism.

Contribution

It provides new insights into the spatial and physical structure of the circumnuclear environment in NGC 1068 through detailed spectral analysis of iron lines.

Findings

Neutral Fe K$eta$/K$ extalpha$ ratio and low Compton shoulder limit suggest a non-homogeneous, partly optically thin medium.

Broad Fe XXV and Fe XXVI lines indicate a high-velocity, ionized outflow component.

Iron emission lines reveal a stratified environment with distinct neutral and ionized regions.

Abstract

We analyze a XRISM/Resolve observation of NGC1068, focusing on the Fe K$\alpha$ and Fe K$\beta$ fluorescent lines and on the Fe XXV and Fe XXVI emission complexes. Line centroid energies, intrinsic widths, flux ratios, and constraints on the Compton shoulder are derived through local spectral fitting, and compared with atomic calculations and theoretical predictions. The centroid energies of the Fe K$\alpha$ and Fe K$\beta$ lines tightly constrain the emitting material to be neutral or near-neutral. The observed Fe K$\beta$/K$\alpha$ ratio, together with the stringent upper limit on the Compton shoulder ($\lesssim$8--11% of the core flux), disfavour reflection dominated by a homogeneous, classical Compton-thick medium, indicating that most of the neutral Fe K$\alpha$ emission arises in optically thin or moderately Compton-thick gas. The Fe XXV and Fe XXVI emission lines exhibit remarkably large velocity widths, of several thousand km~s$^{-1}$. These broad profiles closely resemble the integrated optical and infrared [O III] and [O IV] lines associated with the large-scale biconical outflow, and are naturally interpreted as the X-ray signature of a more highly ionized, faster, and more spatially confined phase of the same outflow. The iron-K emission of NGC1068 reveals a stratified circumnuclear environment in which neutral and highly ionized components arise in physically distinct regions. The neutral Fe K fluorescence originates predominantly in optically thin or mildly Compton-thick material, despite the persistently Compton-thick line-of-sight obscuration, indicating a geometrically complex cold reprocessor. The highly ionized iron emission lines trace a fast component consistent with a warm bipolar outflow on parsec scales, whose large velocities and inferred energetics suggest that it may represent an efficient channel for feedback in a heavily obscured Seyfert galaxy.