A novel “spectral-ratio model fitting” to resolve complicated X-ray spectral variations in active galactic nuclei

Abstract

Radiation-magnetohydrodynamic simulation of the active galactic nuclei predicts the presence of the 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 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 in 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).