Polarization properties of changing-look active galactic nuclei: NGC 1365 and NGC 2992

TL;DR

Changing-look AGNs offer a testbed for BLR visibility and polarization mechanisms. The authors combine spectropolarimetry and imaging polarimetry of two CLAGNs, NGC 1365 and NGC 2992, with known radio jet axes to anchor polarization geometry. For NGC 1365, polarization parallel to the jet arises from scattering, but blueward dichroic extinction in the bar rotates the angle; after removing this component, the intrinsic scattering polarization remains wavelength-independent and consistent with $p$- and $\theta$-values expected for electron scattering. In NGC 2992, dichroic extinction in the highly inclined disk dominates the polarization, keeping the angle constant across AGN states and implying that the faint broad lines in polarized flux are direct, not scattered, light, with high line polarization explained by differential dilution. Overall, host-disk dichroism and dilution effects strongly shape CLAGN polarization, highlighting the need for time-resolved, geometry-aware polarimetry across changing-look cycles.

Abstract

Changing-look active galactic nuclei (CLAGNs) represent a rare class of AGNs that undergo transitions from type 1 (characterized by the presence of broad emission lines in their spectra) to type 2 (absence of broad emission lines) or vice versa, over timescales ranging from months to years. Since normal type 1 and type 2 AGNs are known to show different polarization properties, detailed investigations of the CLAGN polarization can shed light on the underlying mechanisms responsible for the changing-look phenomenon. We present new (spectro)polarimetric observations of two changing-look AGNs located in the core of the inclined spiral galaxies NGC 1365 and NGC 2992. Both AGNs are radio emitters, thereby enabling a comparison of their polarization to the radio jet axis, which defines the accretion disk geometry. In the case of NGC 1365, the AGN shows polarization characteristics consistent with those observed in type 1 Seyferts, in particular polarization parallel to the radio jet. This intrinsic polarization is modified by the wavelength-dependent dichroic extinction that occurs in the galaxy bar and that rotates the polarization angle at the shortest wavelengths. NGC 2992, on the other hand, is so inclined that dichroic dust extinction in the disk completely dominates the polarization of the AGN, thus overwhelming any polarization due to scattering. Consequently, the polarization properties remain essentially constant between the different AGN states, and the faint broad lines observed in the polarized flux are most likely not scattered light. Differential dilution between the continuum and the narrow-line polarizations can explain the unusually high polarization measured in the emission lines.

Figures (10)

• Figure 1: Visible image of the central part of NGC 1365 obtained during the FORS2 observations. The field size is 40$\times$ 40. The directions north and east are indicated. The position of the slit, centered on the AGN, is also shown.
• Figure 2: Same as Fig. \ref{['fig:ngc1365']}, but for NGC 2992.
• Figure 3: Spectropolarimetry of NGC 1365N. From top to bottom: Direct and polarized fluxes in arbitrary units, debiased polarization degree in percent, normalized Stokes parameters $q$ and $u$ in percent, and polarization position angle in degrees (from which 90 were subtracted for clarity of the plot). The spectra in green were binned over 30 spectral pixels (100 Å ) to increase the signal-to-noise ratio.
• Figure 4: Fit of narrow- and broad-line profiles to the H$\beta$ and [Oiii] emission lines observed in the direct spectrum of NGC 1365N. The continuum and the Feii emission (in green) were previously subtracted. The observed and the modeled spectra are superimposed and shifted along the y-axis for clarity. The flux density is given in arbitrary units.
• Figure 5: Same as Fig. \ref{['fig:ngc1365_hbeta']}, but for the H$\alpha$ and [Nii] emission lines.