Delving into the depths of NGC 3783 with XRISM

IV. Mapping of the accretion flow with Fe Kα emission lines

¹ Leiden Observatory, Leiden University PO Box 9513 2300 RA Leiden, The Netherlands
² SRON Netherlands Institute for Space Research Niels Bohrweg 4 2333 CA Leiden, The Netherlands
³ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
⁴ Lawrence Livermore National Laboratory Livermore CA 94550, USA
⁵ ESA European Space Research and Technology Centre (ESTEC) Keplerlaan 1 2201 AZ Noordwijk, The Netherlands
⁶ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA
⁷ Center for Astrophysics | Harvard & Smithsonian 60 Garden St. Cambridge MA 02138, USA
⁸ Science Research Education Unit, University of Teacher Education Fukuoka, Munakata Fukuoka 811-4192, Japan
⁹ Department of Astronomy, University of Michigan Ann Arbor MI 48109, USA
¹⁰ Department of Physics and Astronomy, James Madison University Harrisonburg VA 22807, USA
¹¹ Department of Physics, Technion Haifa 32000, Israel
¹² INAF – Osservatorio Astrofisico di Arcetri Largo Enrico Fermi 5 I-50125 Florence, Italy
¹³ University of Maryland College Park, Department of Astronomy College Park MD 20742, USA
¹⁴ NASA Goddard Space Flight Center (GSFC) Greenbelt MD 20771, USA
¹⁵ Center for Research and Exploration in Space Science and Technology, NASA GSFC (CRESST II) Greenbelt MD 20771, USA
¹⁶ ESA European Space Astronomy Centre (ESAC) Camino Bajo del Castillo s/n 28692 Villanueva de la Cañada Madrid, Spain
¹⁷ Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA) Kanagawa 252-5210, Japan
¹⁸ Graduate School of Science and Engineering, Kagoshima University Kagoshima 890-8580, Japan
¹⁹ Department of Astronomy, Kyoto University Kitashirakawa-Oiwake-Cho Sakyo Kyoto 606-8502 Kinki, Japan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 October 2025
Accepted: 8 December 2025

Abstract

Using XRISM/Resolve 439 ks time-averaged spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 3783, we investigated the nature of the Fe Kα 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 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°. 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_ISCO and derived a lower limit on the spin of a ≥ 0.29 at the 3σ confidence level. A Compton shoulder is detected in our data as well as a 2–3σ detection of the Cr Kα and Ni Kα lines.