The X-ray Disk/Wind Degeneracy in AGN

TL;DR

This paper reveals that relativistic Fe K emission lines from accretion disks and disk winds can produce indistinguishable profiles, complicating the interpretation of black hole accretion processes in X-ray spectra.

Contribution

It demonstrates the degeneracy between disk reflection and wind scattering in Fe K line profiles and highlights the biases in parameter estimation when using single-process models.

Findings

Disk and wind processes can produce identical Fe K line profiles.

Single-process models lead to systematic biases in key parameter estimates.

Including high-energy data can help mitigate degeneracy effects.

Abstract

Relativistic Fe K emission lines from accretion disks and from disk winds encode key information about black holes, and their accretion and feedback mechanisms. We show that these two processes can in principle produce indistinguishable line profiles, such that they cannot be disentangled spectrally. We argue that it is likely that in many cases both processes contribute to the net line profile, and their relative contributions cannot be constrained purely by Fe K spectroscopy. In almost all studies of Fe K emission to date, a single process (either disk reflection or wind Compton scattering) is assumed to dominate the total line profile. We demonstrate that fitting a single process emission model (pure reflection or pure wind) to a hybrid line profile results in large systematic biases in the estimates of key parameters, such as mass outflow rate and spin. We discuss various strategies to mitigate this effect, such as including high energy data covering the Compton hump, and the implications for future X-ray missions.