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

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 ≲ Lbol ≲ 1049 erg s−1), black hole mass (106 ≲ MBH/M⊙ ≲ 1010), and Eddington ratio (−2.7 ≲ log λEdd ≲ 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 υout, with the υout − 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 υout 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.