Delving into the depths of NGC 3783 with XRISM IV. Mapping of the accretion flow with Fe K$α$ emission lines

TL;DR

This XRISM/Resolve study of NGC 3783 dissects the Fe Kα complex with unprecedented spectral resolution, separating a narrow core, an intermediate-width component, and a broad relativistic emission feature. By testing neutral, He-like, and H-like ionization states in a relativistic disk-line framework, the analysis constrains the inner disk radius to a few $R_{\rm g}$ and yields a robust lower limit on black hole spin, $a \ge 0.29$ at 3σ when the inner radius is tied to the ISCO; inclination varies with ionization, illustrating degeneracies between geometry and ionization state. The results also detect a marginal Compton shoulder and weak Cr Kα and Ni Kα lines, consistent with solar abundances and distant reflector material. Overall, the study advances our understanding of the accretion flow structure in NGC 3783, demonstrates the power of high-resolution Fe Kα spectroscopy for spin and geometry constraints, and highlights the need for joint broadband data to break degeneracies and fully map the reflection spectrum.

Abstract

Using XRISM/Resolve $439 \, \rm ks$ time-averaged spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 3783, we investigate the nature of the Fe K$α$ emission line at 6.4 keV, the strongest and most common X-ray line observed in AGN. Even the narrow component of the line is resolved with evident Fe K$α_{1}$ (6.404 keV) and K$α_{2}$ (6.391 keV) contributions in a 2:1 flux ratio, fully consistent with a neutral gas with negligible bulk velocity. The narrow and intermediate-width components have a full-width at half maximum (FWHM) of 350 $\pm$ 50 km/s and $3510 \pm 470 \, \rm km/s$, respectively, suggesting that they arise in the outer disk/torus and/or BLR. We detect a $10\%$ excess flux around 4 $-$ 7 keV that is not well described by a symmetric Gaussian line, but is consistent with a relativistically broadened emission line. In this paper, we take the simplest approach to model the asymmetric line as a single emission line (assuming either neutral, He-like or H-like iron) convolved with a relativistic disk line model. As expected, the inferred inclination angle is highly sensitive to the assumed ionization state, and ranges between $i=17-44^{ \circ}$. This model also constrains the black hole spin via the extent of the red wing: the required gravitational redshift in the fitted disk-line profile disfavors a non-spinning (Schwarzschild) black hole. The derived inner radius is close to the radius of the innermost stable circular orbit $r_{\rm ISCO}$ and strongly correlated with the black hole spin. To better constrain the spin, we fix the inner radius at $r_{\rm ISCO}$ and derive a lower limit on the spin of $a \ge 0.29$ at the 3 $σ$ confidence level. A Compton shoulder is detected in our data as well as a $2-3 \, σ$ detection of the Cr K$α$ and Ni K$α$ lines.

Figures (9)

• Figure 1: XRISM/Resolve residual spectra for each added model component one by one: (a) power-law; (b) photoabsorbed power-law; (c) all data, with narrow absorption lines excluded as described in the text; (d) narrow Fe core added; (e) intermediate neutral Fe component added; (f) residual broad emission feature, re-binned to 60 eV to better reveal the broad emission structure.
• Figure 2: The transmission of five warm absorber components using slab model with Resolve data. In our current emission-line and continuum modelling, we include only the total continuum absorption from the warm absorbers (shown in black).
• Figure 3: XRISM/Resolve residual plot of NGC 3783 after modelling the Fe K$\alpha$ core with three emission components. The residuals are modelled with the yellow line in the Figure \ref{['fig_spec']}.
• Figure 4: XRISM/Resolve time-averaged spectrum of NGC 3783 with the best-fitting model A shown as red solid line. The top two panels show the Resolve background-subtracted spectrum with individual model components and the corresponding fit residuals. The fit residuals are defined as (data $-$ model) / model. In our fitting, we used all data points in black excluding the strong absorption line data points (in gray) for modelling continuum and emission lines independently. For a clarity of display the spectrum in the top panel, it is additionally binned up to 8 eV. The bottom two panels provide a close-up view of the Fe K band and its fit residuals at the model optimal bin size of 2 eV. In both panels, the strongest emission features are labelled. The individual components are plotted by dashed lines representing the power law (cyan), iron narrowest (magenta) and intermediate (purple) component, relativistic iron line (blue), Compton shoulder (orange), Cr K$\alpha$ and Ni K$\alpha$ (green). The background model is depicted by the gray curve.
• Figure 5: XRISM/Resolve spectra of NGC 3783 fitted with three different models: Model A (red), Model B (blue), and Model C (green). The top two panels illustrate the subtle differences among these models. Residuals are calculated as (data $-$ model) / model, similar to Fig. \ref{['fig_spec']}. The bottom panel shows a close-up view of the complex spectral structure in the 6.75 $-$ 7.1 keV energy range. Spectral bins containing narrow absorption features are included in the spectral plot but excluded from the residual plot.