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Tracking the state transitions in changing-look active galactic nuclei through their polarized-light echoes

Abstract : Context. Variations in the mass accretion rate appear to be responsible for the rapid transitions in spectral type that are observed in increasingly more active galactic nuclei (AGNs). These objects are now labeled “changing-look” AGNs and are key objects for understanding the physics of accretion onto supermassive black holes.Aims. We aim to complement the analysis and interpretation of changing-look AGNs by modeling the polarization variations that can be observed, in particular, polarized-light echoes.Methods. We built a complex and representative model of an AGN and its host galaxy and ran radiative transfer simulations to obtain realistic time-dependent polarization signatures of changing-look objects. Based on actual data, we allowed the system to become several times fainter or brighter within a few years, assuming a rapid change in accretion rate.Results. We obtain time-dependent polarization signatures of distant high-luminosity (quasars) and nearby low-luminosity (Seyferts) changing-look AGNs for a representative set of inclinations. We predict the evolution of the continuum polarization for future polarimetric campaigns with the goal to better understand the physics at work in these objects. We also investigate highly inclined AGNs that experience strong accretion rate variations without appearing to change state. We apply our modeling to Mrk 1018, the best-documented case of a changing-look AGN, and predict a variation in its polarization after the recent dimming of its continuum.Conclusions. We demonstrate that polarization monitoring campaigns that cover the transitions that are observed in changing-look AGNs might bring crucial information on the geometry and composition of all the reprocessing regions within the nucleus. In particular, specific features in the time variation of the polarization position angle can provide a new and efficient method for determining AGN inclinations.Key words: galaxies: active / galaxies: Seyfert / polarization / quasars: general / radiative transfer / scattering
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F. Marin, D. Hutsemékers. Tracking the state transitions in changing-look active galactic nuclei through their polarized-light echoes. Astronomy and Astrophysics - A&A, EDP Sciences, 2020, 636, pp.A23. ⟨10.1051/0004-6361/201936123⟩. ⟨hal-02542861⟩

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