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

TL;DR

This study analyzes XRISM spectra of NGC 3783 to characterize Fe Kα emission lines, revealing details about the accretion flow, black hole spin, and the origin of different line components in the AGN.

Contribution

It provides the first detailed modeling of the relativistically broadened Fe Kα line in NGC 3783, constraining the black hole spin and accretion disk geometry using high-resolution XRISM data.

Findings

Resolved narrow Fe Kα components with flux ratio 2:1.

Detected relativistically broadened Fe line indicating a spinning black hole.

Constrained black hole spin to be at least 0.29 at 3σ confidence.

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$\alpha$ 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$\alpha_{1}$ (6.404 keV) and K$\alpha_{2}$ (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 $\sigma$ confidence level. A Compton shoulder is detected in our data as well as a $2-3 \, \sigma$ detection of the Cr K$\alpha$ and Ni K$\alpha$ lines.