UV and optical polarization modeling of thermal active galactic nuclei : impact of the narrow line region

TL;DR

This study models the impact of the narrow line region on the optical/UV polarization of active galactic nuclei, revealing its significant role in altering polarization signatures and aiding the interpretation of spectropolarimetric observations.

Contribution

It introduces an upgraded AGN polarization model including the NLR, demonstrating its influence on polarization features and differences between type-1 and type-2 AGN.

Findings

NLR can suppress the polarization dichotomy between AGN types.

NLR decreases polarization percentage and changes spectral shape in type-2 AGN.

Including NLR helps explain observed spectropolarimetry of type-2 AGN.

Abstract

In this research note, we start exploring the influence of the narrow line region (NLR) on the optical/UV continuum polarization of active galactic nuclei (AGN). We have upgraded our previous 3-component model of a thermal Seyfert nucleus that was composed of an equatorial, optically thin electron disc, a circumnuclear dusty torus, and a pair of collimated, optically thin electron winds (Marin et al.2012b). We have added a dusty extension with low optical depth to the outflows to account for continuum scattering and absorption inside the NLR. A spectropolarimetric comparison between our AGN models with and without NLR reprocessing is carried out. It turns out that the NLR can alter and even suppress the observed polarization dichotomy between type-1 and type-2 AGN. For type-2 AGN, it also significantly decreases the expected percentage of polarization and alters its spectral shape. While the NLR makes it more difficult to reproduce the observed polarization in type-1 objects, it helps to explain spectropolarimetry observations of type-2 objects. Further studies including clumpy media need to be carried out.