Spectropolarimetry of the changing-look active galactic nucleus NGC 1566 and its potential link to supermassive black hole binaries

Abstract

The AGN NGC~1566 is known to present dramatic and regular spectral shape changes, associated with the appearance and disappearance of broad emission lines. The underlying mechanism responsible for such changes is yet to be identified, but occultation, eccentric accretion disks, turbulent disk-dominated broad line regions (BLRs) or binary supermassive black holes have been hypothesized. Because the scenarios used to explain the variable spectral shapes of NGC~1566 each have a specific geometric configuration, we used the VLT/FORS2 instrument to obtain nine 3500-10\,000~Å\, polarized spectra of the source between August 2 and September 21, 2025. We caught the AGN in a type-2 state, i.e., without any broad component in total nor polarized fluxes. Its low and wavelength-independent polarization degree (and angle) above 4000~Å\, argues against occultation of the BLR and is consistent with a significant weakening or disappearance of the BLR. The polarized spectrum reveals a strong rise of polarization in the blue band, likely echoing the 2018 outburst of the AGN. The temporal variability of the total flux continuum but the steadiness of the line profiles demonstrate that the object is viewed close to pole-on, irrespective of its spectral type at the time of observation. Relative to archival data, NGC 1566 shows significant variability in polarization degree, angle, and wavelength dependence. Even more surprisingly, NGC~1566 behaves opposite to the basic predictions of the unified model: its polarization angle is perpendicular to the AGN polar axis and its polarization degree is higher when in a brighter, type-1 phase. The results reported above contradict occultation and binary supermassive black hole hypotheses, rather supporting accretion-driven photoionization/structural changes in the internal accretion flow and the BLR.

Figures (11)

• Figure 1: Total-flux spectra obtained using different slit lengths and different combination methods (average and median), applied to the merged dataset of NGC 1566. Top panel: Fluxes for the FORS2 300V grism. Bottom: Same but for the 300I grim (+ OG590 filter).
• Figure 2: NGC 1566's Stokes parameter spectra (Q/I and U/I) extracted using different slit lengths and combination methods (average and median), integrated over continuum bands free of strong emission lines. Top panel: 300V grism data. Bottom panel: 300I grism (+ OG590 filter).
• Figure 3: Interstellar polarization (ISP) pattern around NGC 1566, based on star polarization from the compilation of Panopoulou2025. The red cross at the center marks the position of the AGN, while the black squares mark the position of various stars within a box with sides of 20 degrees centered around the AGN. Arrows indicate the stellar polarization vectors, with color encoding their polarization degree and orientation corresponding to their polarization angle.
• Figure 4: VLT/FORS2 spectropolarimetry of NGC 1566. Top panel: Total flux spectrum (in 10$^{-16}$ erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$). The most prominent telluric absorption lines are indicated in purple, the calcium triplet absorption lines resulting from host starlight are indicated in red, and the most prominent emission lines are labeled in black. The 300I spectrum has been offset to match the 300V spectrum by applying a correction factor of 0.86 to account for the different observing conditions and grisms. Second panel: Polarized flux; that is, the multiplication of the total flux with the polarization degree, $P$. Third panel: Linear polarization degree, $P$. Bottom panel: Polarization position angle, $\theta$. Spectra are shown at native spectral resolution (no binning). Observational errors are indicated in transparent gray for each spectral bin.
• Figure 5: Fits to the observed spectrum of NGC 1566 using the starlight code CidFernandes2005. In black is the observed total flux spectrum (as shown in Fig. \ref{['Fig:Data']}, top panel), in red is the synthetic spectrum obtained with the code (that is composed of the stellar component in orange and of the AGN component in yellow), and in green are the residuals. Negative fluxes in the residuals are due to the uncorrected telluric absorption lines.