Variable Nature of Magnetically-Driven Ultra-Fast Outflows

TL;DR

This study models the variable ultra-fast outflows in quasar PDS 456 using magnetohydrodynamic winds, revealing magnetic forces alone can drive these outflows and reproduce observed spectral features and correlations.

Contribution

It introduces a MHD-driven wind model to explain UFO variability and demonstrates magnetic acceleration without radiative pressure in PDS 456.

Findings

UFO velocity correlates with X-ray luminosity.

Equivalent width anti-correlates with X-ray strength.

Magnetic forces can drive ultra-fast outflows without radiation pressure.

Abstract

Among a number of active galactic nuclei (AGNs) that drive ionized outflows in X-rays, a low-redshift (z = 0.184) quasar, PDS 456, is long known to exhibit one of the exemplary ultra-fast outflows (UFOs). However, the physical process of acceleration mechanism is yet to be definitively constrained. In this work, we model the variations of the Fe K UFO properties in PDS 456 over many epochs in X-ray observations in the context of magnetohydrodynamic (MHD) accretion-disk winds employed in our earlier studies of similar X-ray absorbers. We applied the model to the 2013/2014 XMM-Newton/NuSTAR spectra to determine the UFO's condition; namely, velocity, ionization parameter, column density and equivalent width (EW). Under some provisions on the dependence of X-ray luminosity on the accretion rate applicable to near-Eddington state, our photoionization calculations, coupled to a 2.5-dimensional MHD-driven wind model, can further reproduce the observed correlations of the UFO velocity and the anti-correlation of its EW with X-ray strength of PDS 456. This work demonstrates that UFOs, even without radiative pressure, can be driven as an extreme case purely by magnetic interaction while also producing the observed spectrum and correlations.