XRISM/Resolve reveals the complex iron structure of NGC 7213: Evidence for radial stratification between inner disk and broad-line region

TL;DR

This study uses XRISM Resolve, XMM-Newton, NuSTAR, and SOAR optical data to resolve the Fe K region in NGC 7213 with unprecedented detail. Through continuum, line-profile, and MCMC analyses, it reveals a multi-component Fe K$\alpha$ complex: a narrow core from distant, Compton-thin material at the dust sublimation radius and a broad, disk-like component from $\sim100\,R_{g}$, plus highly ionized Fe$^{25}$/Fe$^{26}$ lines from intermediate radii bridging the inner disk and the optical BLR. The findings support a radially and vertically stratified accretion structure, with a weak torus indicating dissipation at the current low accretion rate $\lambda_{\rm Edd}\simeq0.001-0.01$, placing NGC 7213 between LLAGN and Seyfert regimes. These results have implications for AGN unification and the continuity of disk, BLR, and torus structure across feeding rates. The work demonstrates XRISM's power to map the innermost AGN regions and motivates multi-epoch, multi-wavelength follow-ups to test photoionization and dynamical models of disk atmospheres and winds.

Abstract

We present the first high-resolution X-ray spectrum of NGC 7213 obtained with XRISM/Resolve, supported by simultaneous XMM-Newton, NuSTAR, and SOAR optical data. The XRISM spectrum resolves the neutral Fe\,K$α$ into two components: a narrow core ($\rm FWHM = 650_{-220}^{+240}\,\rm km\,s^{-1}$) consistent with emission at the dust sublimation radius, and a broader, asymmetric line best described by disk-like emission from $\sim 100\,\rm R_{g}$. This disk component mirrors the profile of the double-peaked H$α$ line observed in the optical. In addition, we detect broadened Fe XXV and Fe XXVI emission lines whose inferred locations bridge the gap between the inner disk and the optical broad-line region. The weak narrow Fe K$α$ equivalent width ($\rm EW = 32 \pm 6\,eV$) and absence of a Compton hump imply a low-covering-fraction, Compton-thin torus. Together, these results reveal a radially stratified structure in NGC 7213, spanning nearly four orders of magnitude in radius, and place the source in an intermediate accretion state ($\rm λ_{Edd} = 0.001-0.01$) where the inner disk and BLR remain, while the torus shows signs of dissipation.

Figures (12)

• Figure 1: Top panel: XRISM/Resolve (black), XMM-Newton/EPIC-pn and NuSTAR spectra modeled with an absorbed power law with a high-energy cutoff. We also show the corresponding residuals. Bottom panel: The high-energy cutoff versus photon index confidence regions at the 68%, 95%, and 99% levels (black, red, and blue, respectively).
• Figure 2: Data to continuum ratio of the XRISM/Resolve spectrum in the Fe lines region in the observed frame. The red dashed lines show the expected energies for the low-ionization Fe K$\alpha_{1,2}$, and K$\beta$ lines and the high-ionization Fe25 ($w, x, y, z$) and Fe26 K$\alpha_{1,2}$ lines.
• Figure 3: Modeling the XRISM/Resolve spectrum (focusing on the Fe region). In addition to the continuum (blue dotted line), we also show the narrow and broad Fe K$\alpha$ lines (yellow and grey, respectively), the Fe25 emission lines (orange), the Fe26 lines (red), and two Gaussian emission lines added at 6.45 keV and 6.75 keV (green). The left panel shows the best-fit assuming that the broad Fe K$\alpha$ is described with a zFeklor model smoothed with a Gaussian profile. The right panel shows the best-fit assuming that the broad Fe K$\alpha$ is described with a Diskline model. The results in the right panel are based on MCMC analysis. The solid lines and the shaded region correspond to the median profile and the 68% and 95% credibility envelopes of each spectral component, respectively.
• Figure 4: SOAR spectrum of NGC 7213 in the H$\alpha$ region. We fit the data assuming a power-law continuum and a series of Gaussian emission lines to model the narrow H$\alpha$, a broad symmetric H$\alpha$ line, two broad red-/blueshifted H$\alpha$ lines, and narrow [N2] $\lambda\lambda6548,6583$ and [S2] $\lambda\lambda6716,6731$ lines. In addition, the data required broad and blueshifted [N2] $\lambda\lambda6548,6583$ and [S2] $\lambda\lambda6716,6731$, as well as a low-amplitude red-/blue-shifted doublet at $\rm \sim 6682\,\AA$ and $\rm \sim 6472\,\AA$, respectively, with an intermediate broadening of $\sim 900-1000\,\mathrm{km}\,\mathrm{s}^{-1}$.
• Figure 5: Left: The broad double-peaked H$\alpha$ profile of NGC 7213 in velocity space modeled by a diskline with an additional Gaussian smoothing. The negative/positive sign corresponds to blue-/redshift with respect to 6562.8 Å. Right: Confidence contours of the disk inclination versus inner radius obtained by modeling the broad Fe K$\alpha$ line in the XRISM/Resolve spectrum. Shaded regions enclose the 68%, 90%, 95% posterior probability. The solid contours correspond to the contours derived from a grid of $\Delta C = 2.3, 4.6,$ and 10 (black, red, and blue, respectively) which would correspond to 68%, 90%, and 99.5% confidence level in the $\chi^2$ approximation for two parameters of interest. The green circle shows the best-fit obtained by modeling the broad double-peaked H$\alpha$ profile on the left.