Modelling the X-ray polarimetric signatures of complex geometry: The case study of the 'changing look' active galactic nucleus NGC 1365

Abstract

'Changing look' active galactic nuclei (AGN) are a subset of Seyfert galaxies characterized by rapid transitions between Compton-thin and Compton-thick regimes. In their Compton-thin state, the central engine is less obscured; hence, spectroscopy or timing observations can probe their innermost structures. However, it is not clear if the observed emission features and the Compton hump are associated with relativistic reflection on to the accretion disc, or complex absorption by distant, absorbing gas clouds passing by the observer's line of sight. Here, we investigate these two scenarios under the scope of X-ray polarimetry, providing the first polarization predictions for an archetypal 'changing look' AGN: NGC 1365. We explore the resulting polarization emerging from lamp-post emission and scattering off an accretion disc in the immediate vicinity of a supermassive black hole. The computed polarization signatures are compared to the results of an absorption-dominated model, where high column density gas partially covers the central source. While the shape of the polarization spectrum is similar, the two models differ in net polarization percentage, with the relativistic reflection scenario producing significantly stronger polarization. Additionally, the variation of the polarization position angle is distinctly different between both scenarios: the reflection-dominated model produces smooth rotations of the polarization angle with photon energy, whereas circumnuclear absorption causes an orthogonal switch of the polarization angle between the soft and hard X-ray bands. By comparing the predicted polarization of NGC 1365 to the detectability levels of X-ray polarimetry mission concepts proposed in the past, we demonstrate that with a large, soft X-ray observatory or a medium-sized mission equipped with a hard (6-35 keV) polarimeter, the correct interpretation would be unambiguous. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.