Using XRISM/Resolve 439, ̊m ks time-averaged spectra of the well-known Seyfert-1. 5 active galactic nucleus (AGN) in NGC 3783, we investigated the nature of the emission line at 6. 4 keV, the strongest and most common X-ray line observed in AGNs. Even the narrow component of the line is resolved with evident Fe Kα₁ (6. 404 keV) and Kα₂ (6. 391 keV) contributions with 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 of 350 ± 50 km/s and 3510 ± 470, ̊m km/s, respectively, suggesting that they arise in the outer disk/torus and/or broad line region. We detect a 10% excess flux around 4 - 7 keV that is not described well by a symmetric Gaussian line but is consistent with a relativistically broadened emission line. In this study we took the simplest approach, modelling the asymmetric line as a single emission line (assuming neutral, He-like, or H-like iron) convolved with a relativistic disk line model. As expected, the inferred inclination angle (i) is highly sensitive to the assumed ionisation state and ranges between 17 and 44^ ̧irc. 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 disfavours a non-spinning (Schwarzschild) black hole. The derived inner radius is close to the radius of the innermost stable circular orbit (r_ ISCO) and is strongly correlated with the black hole spin. To better constrain the spin, we fixed the inner radius to r_̊m ISCO and derived a lower limit on the spin of a 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 Kα and Ni Kα lines.
Li et al. (2026) studied this question.