A XRISM View of the Iron Line Complex in NGC 1068: Rethinking the Prototypical Compton-Thick AGN

Abstract

We analyze an XRISM/Resolve observation of NGC1068, focusing on the Fe K$α$ and Fe K$β$ 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$α$ and Fe K$β$ lines tightly constrain the emitting material to be neutral or near-neutral. The observed Fe K$β$/K$α$ ratio, together with the stringent upper limit on the Compton shoulder ($\lesssim$8--11% of the core flux), disfavour reflection dominated by a homogeneous, classical Compton-thick medium, indicating that most of the neutral Fe K$α$ 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.

Figures (6)

• Figure 1: XRISM/Resolve spectrum of NGC 1068 showing only emission lines detected at $\geq 99\%$ confidence in the blind line scan (Sect. \ref{['sec:spectralanalysis']}). For clarity, the detected lines are displayed in two separate energy bands, which encompass all features meeting the detection criterion. Vertical dashed lines mark the centroid energies of the detected features, which are labeled in the figure (see Table \ref{['tab:lines']}).
• Figure 2: Local fits to the neutral fluorescence lines in the XRISM/Resolve spectrum of NGC 1068. Top: Fe K$\alpha$ complex (6.0--6.5 keV, rest frame). Black points show the data and the black histogram the best-fitting model, consisting of a narrow core (red) and a broad base (blue; see text), added to the baseline continuum (grey). Magenta dashed curves show bound electron-scattering Compton-shoulder profiles with fluxes of 10 and 20% of the core. Bottom left: Fe K$\beta$ band. Bottom right: Ni K$\alpha$ band. Vertical dotted lines mark the nominal centroid energies of the neutral transitions.
• Figure 3: XRISM/Resolve spectrum of NGC 1068 with the broad Fe xxv He$\alpha$ and Fe xxvi Ly$\alpha$ emission lines. The spectrum is shown in the rest frame and is fitted with the outflow model described in Sect. 4.2.1. The data are plotted in black, while the model components for the Fe xxv and Fe xxvi lines are shown in blue; the total model is shown in grey. The neutral Fe K$\beta$ line, shown in red, is also included in the fit. Vertical dotted lines mark the laboratory energies of the main transitions.
• Figure 4: Upper panel: Velocity shifts of the Fe K$\beta$ centroid energy relative to the neutral reference of Holzer1997, shown as a function of ionization stage. Blue diamonds show the shifts predicted by the atomic calculations of Palmeri2003, while the grey horizontal band indicates the value measured in the XRISM/Resolve spectrum. The green square and red triangle mark the experimental neutral Fe K$\beta$ centroid energies from Holzer1997 and Bearden1967, respectively. The measured centroid is consistent with neutral iron and rules out, within the statistical uncertainties, ionization stages higher than Fe v. Lower panel: Fe K$\beta$/K$\alpha$ ratio as a function of ionization stage. Black circles show the values predicted by KaastraMewe1993, while the light blue shaded region indicates the envelope spanned by the theoretical calculations of Palmeri2003. The green square marks the experimental neutral Fe K$\beta$/K$\alpha$ ratio from Holzer1997, and the horizontal band indicates the ratio measured in the XRISM/Resolve spectrum. The observed ratio is consistent, within the uncertainties, with the theoretical expectation for neutral Fe in the optically thin limit, although marginally lower than the nominal value.
• Figure 5: High-velocity bi-polar outflow model adopted to reproduce the broad Fe xxv and Fe xxvi lines observed with XRISM. The intrinsic line spectrum was calculated with XSTAR, the integrated velocity profile was calculated with the SKIRT code.