[Paper Review] Changing-look Seyfert galaxies with optical linear polarization measurements

Frédéric Marin, Damien Hutsemékers+1 more|arXiv (Cornell University)|Sep 6, 2019

Astronomy and Astrophysical Research1 references6 citations

TL;DR

This paper advocates for new spectro-polarimetric monitoring campaigns of changing-look Seyfert galaxies (CLAGNs) to distinguish between competing physical mechanisms—such as obscuration, tidal disruption events, or accretion rate changes—driving their rapid spectral transitions. It demonstrates that polarization signatures, particularly time-dependent polarization degree and position angle variations, uniquely diagnose the geometry and dynamics of the inner AGN regions, with only 23 historical polarization measurements available, most from the 1980s–1990s, underscoring an urgent need for modern, repeated observations.

ABSTRACT

In this lecture note, we make the case for new (spectro)polarimetric measurements of "changing-look" AGNs (CLAGNs), a subclass of the AGN family tree that shows long-term (months to years) large flux variability associated with the appearance or disappearance of optical broad emission lines. We discuss how polarization measurements could help to distinguish which of the several scenarios proposed to explain such variations is/are the most likely. We collected past polarization measurements of nearby, Seyfert-like CLAGNs and take stock that almost all polarimetric information we have on those fascinating objects dates from the 80's and 90's. We thus explain how polarization could help us understand the physical processes happening in the first parsecs of CLAGNs and why new polarization monitoring campaigns are strongly needed.

Motivation & Objective

• To identify and analyze the limited archival polarimetric data on changing-look Seyfert galaxies (CLAGNs), revealing a critical data gap in modern observations.
• To demonstrate that polarization measurements—especially time-resolved degree and position angle—can uniquely distinguish between physical mechanisms causing CLAGN transitions.
• To argue that current polarization data, mostly from the 1980s–1990s, are insufficient to constrain the true physical origin of spectral changes.
• To call for new, systematic spectro-polarimetric monitoring campaigns of CLAGNs, especially around the time of spectral transitions.
• To highlight that polarization echoes and reverberation mapping can constrain the geometry and size of the scattering regions in CLAGNs.

Proposed method

• Compilation of historical spectral type transitions and polarization measurements for known CLAGNs from the literature, with tabulated data on spectral types, epochs, and polarization degrees.
• Analysis of polarization signatures expected under three competing scenarios: obscuration by transient clouds, tidal disruption events (TDEs), and accretion rate variations.
• Use of polarized reverberation mapping principles: time delays between direct and scattered light modulate polarization degree and position angle, depending on the scattering geometry.
• Modeling of polarization echoes: when the broad-line region disappears, scattered light from the torus or winds produces long-duration, 90°-rotated polarization position angle signals.
• Comparison of observed polarization behavior (e.g., PPA stability, degree changes) with theoretical predictions for each scenario to infer physical mechanisms.
• Identification of candidate CLAGNs with potential for future polarization monitoring based on spectral variability and prior polarimetric data.

Experimental results

Research questions

• RQ1 What are the unique polarization signatures associated with different physical mechanisms driving CLAGN transitions, such as obscuration, TDEs, or accretion rate changes?
• RQ2 How can time-resolved polarization measurements constrain the geometry and size of the scattering regions in CLAGNs, particularly the inner torus or outflows?
• RQ3 Why is the current archival pool of polarization data on CLAGNs so limited, and what are the consequences for understanding their physical mechanisms?
• RQ4 Can polarization echoes—characterized by a 90° rotation in position angle and delayed polarization degree recovery—be used to measure the inner radius of the obscuring torus?
• RQ5 What observational strategy (e.g., cadence, telescope class) is optimal for detecting polarization changes during CLAGN transitions?

Key findings

• Only 23 representative polarization measurements exist for Seyfert-like CLAGNs, with just three conducted after 2000, indicating a severe data gap in modern polarimetry.
• Among the 23 measurements, only six objects (Mrk 6, NGC 1566, NGC 4151, NGC 7603, Fairall 9, and 3C 390.3) have repeated polarimetric observations, none coinciding with spectral transitions.
• Polarization degree decreases and position angle rotates by 90° when the broad-line region disappears, due to loss of nearby electron-scattering material, with a delayed echo from the torus.
• In the obscuration scenario, polarization is dominated by scattering in polar outflows, leading to stable or slowly varying position angles, unlike the dramatic 90° rotations seen in BELR disappearance.
• Polarization reverberation—measured in NGC 4151 and 3C 390.3—provides a method to map the scattering region geometry and estimate the inner radius of the torus.
• The absence of simultaneous polarization and spectral monitoring during transitions severely limits the ability to distinguish between physical mechanisms, necessitating future coordinated campaigns.