A novel "spectral-ratio model fitting" to resolve complicated AGN X-ray spectral variations

TL;DR

This paper introduces a new spectral-ratio model fitting method to analyze complex AGN X-ray spectral variations, revealing that clumpy absorbers and UFOs likely share a common origin in the hot inner and clumpy outer wind model.

Contribution

The paper presents a novel spectral-ratio model fitting technique to disentangle spectral components and constrain the origin of absorbers and outflows in AGNs.

Findings

Clumpy absorbers are outflowing at ~0.2-0.3c.

Soft spectral variations are mainly due to changes in absorber covering fraction.

Clumpy absorbers and UFOs likely originate from the same wind structure.

Abstract

Contemporary radiation-magnetohydrodynamic simulation of the AGNs predicts presence of the hot and strong accretion disk wind, which gets unstable far from the central region and turns into gas clumps. These inner-wind and outer clumps may be actually observed as the ultrafast outflows (UFOs) and the clumpy absorbers, respectively. We may call this picture as the "hot inner and clumpy outer wind model". Observationally, it is challenging to place constraints on the origin of the UFOs and clumpy absorbers due to complicated spectral variations. We developed a novel method, "spectral-ratio model fitting", to resolve parameter degeneracy of the clumpy absorbers and other spectral components. In this method, the parameters of the absorber in the line of sight are estimated from the ratio of the partially absorbed spectrum to the non-absorbed one. We applied this method to the narrow-line Seyfert 1 galaxy IRAS 13224-3809 observed by XMM-Newton in 2016 for 1.5 Ms, where the source showed extreme spectral variability and complex absorption features. As a result, we found that the soft spectral variation is mostly explained by a change of the covering fraction of the mildly-ionized clumpy absorbers, and that these absorbers are outflowing with such a high velocity that is comparable to that of the UFO (~ 0.2-0.3 c). This result implies that the formation of the clumpy absorbers and the UFO shares the same origin, supporting the "hot inner and clumpy outer wind model".