A XRISM view of the iron line complex in NGC 1068: Rethinking the prototypical Compton-thick AGN

¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy
² European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁴ INAF Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ INAF Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate (LC), Italy
⁶ Department of Physics, Institute for Astrophysics and Computational Sciences, The Catholic University of America, Washington, DC 20064, USA
⁷ Dipartimento di Fisica, Università di Trento, Via Sommarive 14, 38123 Trento, Italy
⁸ INAF Osservatorio di Astrofisica e Scienza dello Spazio di Bologna (OAS), Via Gobetti 93/3, I-40129 Bologna, Italy
⁹ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Roma, Italy
¹⁰ Dipartimento di Fisica, “Sapienza” Università di Roma, Piazzale Aldo Moro 2, I-00185 Roma, Italy
¹¹ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy
¹² Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA
¹³ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹⁴ ICREA, Pg Lluís Companys 23, 08010 Barcelona, Spain
¹⁵ Cahill Center for Astronomy & Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA
¹⁶ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
¹⁷ ASI Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹⁸ European Space Agency (ESA), European Space Astronomy Centre (ESAC), 28691 Villanueva de la Cañada, Madrid, Spain
¹⁹ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
²⁰ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²¹ INAF IASF Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy
²² Osservatorio Astronomico di Brera (INAF), Via E. Bianchi 46 Merate 23807, Italy
²³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, Garching 85748, Germany
²⁴ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
²⁵ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Avenida Ejército Libertador 441 Santiago, Chile
²⁶ INFN – Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
²⁷ Dipartimento di Fisica e Astronomia ‘Augusto Righi’, Università degli Studi di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy

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

Received: 18 February 2026
Accepted: 27 April 2026

Abstract

Context. NGC 1068 has long served as the reference Compton-thick Seyfert 2 galaxy. Decades of X-ray observations have established the presence of both neutral reflection and ionized circumnuclear emission. However, the limited spectral capabilities of previous CCD and grating data in the Fe–K band have prevented a clean separation of these components, thus leaving large uncertainties on key properties, such as the optical depth, structure, and kinematics of the reprocessing and emitting regions, and on their connection to nuclear outflows and feedback.

Aims. We exploited high-resolution X-ray spectroscopy to disentangle the neutral and ionized iron emission in NGC 1068; constrained the physical properties of the cold reflector through line energies, widths, fluorescence ratios, and Compton-shoulder diagnostics; and investigated the origin, kinematics, and potential feedback role of the highly ionized Fe XXV and Fe XXVI emission.

Methods. We analyzed an XRISM/Resolve observation of NGC 1068, focusing on the Fe Kα and Fe Kβ fluorescent lines and on the Fe XXV and Fe XXVI emission complexes. Line centroid energies, intrinsic widths, flux ratios, and constraints on the Compton shoulder were derived through local spectral fitting, and compared with atomic calculations and theoretical predictions.

Results. The centroid energies of the Fe Kα and Fe Kβ lines tightly constrain the emitting material to be neutral or near neutral. The observed Fe Kβ/Kα ratio, together with the stringent upper limit on the Compton shoulder (≲8−11% of the core flux), disfavor reflection dominated by a homogeneous classical Compton-thick medium, indicating that most of the neutral Fe Kα emission arises in optically thin or moderately Compton-thick gas. The Fe XXV and Fe XXVI emission lines exhibit remarkably large velocity widths, of several thousand km s⁻¹. These broad profiles closely resemble the integrated optical and infrared [O III] and [O IV] lines associated with the large-scale biconical outflow, and are naturally interpreted as the X-ray signature of a more highly ionized, faster, and more spatially confined phase of the same outflow.

Conclusions. The iron-K emission of NGC 1068 reveals a stratified circumnuclear environment in which neutral and highly ionized components arise in physically distinct regions. The neutral Fe K fluorescence originates predominantly in optically thin or mildly Compton-thick material, despite the persistently Compton-thick line-of-sight obscuration, indicating a geometrically complex cold reprocessor. The highly ionized iron emission lines trace a fast component consistent with a bipolar outflow on parsec scales, whose large velocities and inferred energetics suggest that it may represent an efficient channel for feedback. If similar ionized outflows and reflection geometries are common in heavily obscured Seyferts, they may constitute an underappreciated aspect of feedback and circumnuclear structure in this class of sources.