Unveiling the physics behind the spectral variations of "changing-look" quasars with optical polarimetry

TL;DR

This study uses optical polarimetry and radiative transfer simulations to distinguish between models explaining spectral changes in 'changing-look' quasars, confirming that in a specific case, the spectral variation is due to accretion rate changes.

Contribution

First comprehensive polarized radiative transfer simulations for all competing models of changing-look quasars, enabling clear differentiation based on optical flux and polarization features.

Findings

Distinct polarization signatures for different models

Confirmed accretion rate change as cause in a specific quasar

Provided a method to distinguish physical scenarios in changing-look quasars

Abstract

A handful of active galactic nuclei (AGN) have shown strong spectral variations in the optical band between epochs that are years apart. The appearance or disappearance of broad emission lines in their spectra completely changes their classification. Since their nucleus orientation cannot change in such short timescales another physical interpretation has to be found. Several scenarios are competing to explain their changing-look nature and, for the first time, we conduct polarized radiative transfer Monte Carlo simulations for all the models. We demonstrate that all interpretations have distinctive features in both total optical flux and continuum polarization such as proposed by Hutsem\'ekers and collaborators. Distinguishing between the different scenarios is thus straightforward. We apply our results on the changing-look quasar J1011+5442 and confirm the conclusions found by Hutsem\'ekers and collaborators: in this specific case, the disappearance of the broad emission lines is due to a change in accretion rate.