A song of lines and winds: Tracing the signatures of AGN outflows in X-rays

¹ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata” Via della Ricerca Scientifica 1 I-00133 Roma, Italy
² INFN – Sezione di Roma “Tor Vergata” Via della Ricerca Scientifica 1 I-00133 Roma, Italy
³ INAF – Osservatorio Astronomico di Roma Via Frascati 33 I-00078 Monte Porzio Catone, Italy
⁴ Dipartimento di Fisica, “Sapienza” Universitàdi Roma Piazzale Aldo Moro 2 I-00185 Roma, Italy
⁵ Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia 41125 Modena, Italy
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Gobetti 101 I-40129 Bologna, Italy
⁷ INAF – Istituto di Astrofisica e Planetologia Spaziali Via del Fosso del Caveliere 100 I-00133 Roma, Italy
⁸ Department of Physics and Astronomy, James Madison University Harrisonburg VA 22807, USA
⁹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales Av. Ejército Libertador 441 Santiago, Chile
¹⁰ Space Science Data Center, Agenzia Spaziale Italiana Via del Politecnico snc 00133 Roma, Italy
¹¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester Manchester M13 9PL, UK

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

Received: 27 June 2025
Accepted: 30 November 2025

Abstract

Context. Ultra-fast outflows (UFOs) are highly ionized, mildly relativistic winds seen in the X-ray spectra of active galactic nuclei (AGNs) and are thought to influence AGN feedback and galaxy evolution.

Aims. In this work, we aim to investigate the UFO signatures by analyzing a large and diverse sample of detections from the literature.

Methods. We compiled a sample of 122 solid (> 2σ c.l.) UFO detections for a total of 57 AGNs, spanning broad ranges in redshift (z ≲ 4), bolometric luminosity (10⁴³ ≲ L_bol ≲ 10⁴⁹ erg s⁻¹), black hole mass (10⁶ ≲ M_BH/M_⊙ ≲ 10¹⁰), and Eddington ratio (−2.7 ≲ log λ_Edd ≲ 0.6). We combined results from both phenomenological and photoionization modeling of the absorption features to characterize correlations among UFO parameters.

Results. We find evidence for a positive correlation between the line width σ, the equivalent width EW, and the outflow velocity υ_out, with the υ_out − σ trend being comparatively weak. This suggests that the broadest absorption lines with the largest EW are signatures of the fastest UFOs. We further demonstrate that the inferred velocity dispersion, often much larger than the uncertainty on the centroid velocity, should be accounted for in studies of wind energetics and scaling relations. We estimate lower limits on the launching radii of UFOs finding a minimum distance consistent with the innermost stable circular orbit (ISCO) of a weakly or non-rotating Schwarzschild black hole. This apparent truncation at smaller radii may reflect physical constraints, such as the presence of the X-ray corona or observational limitations due to line broadening. Additionally, variations in the line width to velocity ratio imply differences in wind geometry and kinematics. For the first time, we also explore the dependence of UFOs on AGN class. Differences in UFO properties between Seyferts and quasars – bridged by narrow-line Seyfert 1 galaxies – are likely driven by intrinsic parameters such as black hole mass and luminosity.

Conclusions. The observed co-variation of υ_out with both σ and EW is consistent with clumpy, multi-component winds propagating through a thermally unstable multiphase medium within the chaotic cold accretion (CCA) cycle. These trends are not unique to a single acceleration mechanism: MHD and line-driven winds remain viable. High-resolution spectroscopy from missions like XRISM and NewAthena is necessary to fully resolve the structure, kinematics, and physical origin of UFOs.