Unveiling the dynamics of the ultra-fast outflow in IRAS 13224-3809 with X-ray spectroscopy

TL;DR

This study reanalyzed extensive X-ray data of IRAS 13224-3809, confirming a variable ultra-fast outflow with velocities over 0.2c, and suggested magnetic driving as the acceleration mechanism, highlighting the wind's complex, multiphase structure.

Contribution

It provides a systematic, multi-model reanalysis of the 2016 X-ray datasets, clarifying the nature and driving mechanisms of the ultra-fast outflow in IRAS 13224-3809.

Findings

Confirmed a strong, variable UFO with velocities >0.2c

Found a velocity-luminosity correlation in the outflow

Evidence for rapid wind acceleration during X-ray flares

Abstract

IRAS 13224-3809 is one of the most intensively studied narrow-line Seyfert 1 galaxies, with a rich literature reporting diverse and sometimes contrasting interpretations of its complex X-ray spectra and variability. Notably, a fast and variable ultra-fast outflow (UFO) was discovered in this source, sparking debate over its nature and driving mechanisms. Motivated by these open questions, we present a systematic, time- and flux-resolved reanalysis of the full 2016 XMM-Newton (1.5 Ms) and NuSTAR (500 ks) datasets, employing careful background treatment and equal-count spectral selections. We uniformly apply three spectral models, including photo-ionized absorption, broad emission, and relativistic reflection, to all intervals. We unambiguously confirm the presence of a strong, variable outflow with velocities exceeding 0.2$c$, and find that models including absorption consistently reveal robust physical trends: a velocity-luminosity correlation of the UFO, persistently large line widths, and no compelling equivalent-width-flux anti-correlation. When emission or reflection components are included, the significance of the absorption features decreases, but significant UFO detections remain in most intervals. We also report clear evidence for rapid acceleration of the wind in response to X-ray flares, with the outflow carrying momentum and kinetic power sufficient to drive an efficient AGN feedback. The observed rapid response favors magnetic driving, analogous to coronal mass ejections, over radiative acceleration. Our results reconcile contrasting previous claims and underline the need for high-resolution spectroscopy to resolve the wind substructure. The observed UFO variability and structure are consistent with a multiphase, clumpy wind produced by thermal and hydrodynamic instabilities, with magnetic reconnection providing the rapid acceleration mechanism.