Circumnuclear Multi-phase Gas in the Circinus Galaxy. V. The Origin of the X-Ray Polarization in the Circinus Galaxy

Abstract

The Imaging X-ray Polarimetry Explorer (IXPE) detected X-ray polarization in the nearest Seyfert 2 galaxy, the Circinus galaxy, for the first time. To reproduce the IXPE results, we computed the degree of polarization based on two types of radiative hydrodynamic simulations: a parsec-scale three-dimensional model and a sub-parsec-scale axisymmetric model with a higher spatial resolution. In a series of papers, we confirmed that these models naturally explain the multi-wavelength observations of the Circinus galaxy from radio to X-rays. We used a Monte Carlo Simulation for Astrophysics and Cosmology code to compute the linear polarization of continuum emission. We found that the degree of polarization based on the parsec-scale radiation-driven fountain model was smaller than that observed with the IXPE. The degree of polarization based on the sub-parsec-scale model depends on the hydrogen number density of the disk (\(d\)), and the degree of polarization obtained from our simulation is consistent with that observed with the IXPE in the case of \(\log d/\mathrm{cm}^{-3} \geq 13\). We investigate where the photons are Compton scattered and imply that the origin of the X-ray polarization in the Circinus galaxy is the outflow inside \(0.01 \ \mathrm{pc}\). In this case, the degree of polarization may change over a timescale of approximately ten years.