[en] Type 1 active galactic nuclei (AGNs) are characterized by both broad and narrow emission lines in their optical spectrum while type 2 AGNs only show narrow emission lines. The “unification model” suggests that type 1 and type 2 AGNs are the same objects viewed under different inclinations, the AGN core being obscured by an equatorial dusty torus in type 2 objects. A key argument in favor of the unification model was the discovery of hidden broad line regions in type 2 AGNs using spectropolarimetry. A handful of high luminosity AGNs (quasars) changing from type 1 to type 2 or vice-versa on timescales of a few years have been recently uncovered, thus challenging the unification model. To explain these spectral changes, two main scenarios have been proposed: a variation of the rate of accretion onto the supermassive black hole, or variable dust absorption due to the motion of individual clouds in a clumpy torus. Observations constraining these scenarios will be discussed, with particular emphasis on polarization measurements.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Hutsemekers, Damien ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astroph. extragalactique et observations spatiales (AEOS)
Language :
English
Title :
Changing-look quasars: a challenge for the AGN unification model
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