The X-ray Disk/Wind Degeneracy in AGN

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. from the poorly-fit sample, with 𝜒 2 𝜈 > 2. Some cases are due to a failure of the fitting algorithm, or complex absorption that our simple Gaussian model cannot describe well, but some are genuine cases of the reflection model being unable to fully describe the line profile. A common failure case is caused by a relatively narrow but double-peaked line profile. In practice, this could likely be better fit by including a distant reflection component in addition to the relativistic component.