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"Changing look" Active Galactic Nuclei (AGN) are a subset of Seyfert galaxiescharacterized by rapid transitions between Compton-thin and Compton-thickregimes. 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 humpare associated with relativistic reflection onto the accretion disc, or complexabsorption by distant, absorbing gas clouds passing by the observer'sline-of-sight. Here, we investigate these two scenarios under the scope ofX-ray polarimetry, providing the first polarisation predictions for anarchetypal "changing look" AGN: NGC 1365. We explore the resulting polarisationemerging from lamp-post emission and scattering off an accretion disc in theimmediate vicinity of a supermassive black hole. The computed polarisationsignatures are compared to the results of an absorption-dominated model, wherehigh column density gas partially covers the central source. While the shape ofthe polarisation spectrum is similar, the two models differ in net polarisationpercentage, with the relativistic reflection scenario producing significantlystronger polarisation. Additionally, the variation of the polarisation positionangle is distinctly different between both scenarios: the reflection-dominatedmodel produces smooth rotations of the polarisation angle with photon energywhereas circumnuclear absorption causes an orthogonal switch of thepolarisation angle between the soft and the hard X-ray bands. By comparing thepredicted polarisation of NGC 1365 to the detectability levels of X-raypolarimetry mission concepts proposed in the past, we demonstrate that with alarge, soft X-ray observatory or a medium-sized mission equipped with a hard (6- 35 keV) polarimeter, the correct interpretation would be unambiguous.

Modelling the X-ray polarimetric signatures of complex geometry: the case study of the "changing look" AGN NGC 1365