A new type-1 transition in NGC 1346 revealed by (spectro)polarimetry

TL;DR

This study reports the first spectropolarimetric observations of NGC 1346, revealing a spectral transition back to a type-1 active galactic nucleus with evidence of a re-illuminated broad-line region or a tidal disruption event.

Contribution

It provides the first detailed spectropolarimetric evidence of a changing-look AGN transitioning back to type-1, highlighting the importance of polarization and spectral analysis in understanding AGN variability.

Findings

NGC 1346 reverted to type-1 spectral state after 2022.

Broad H$eta$ and H$ extalpha$ lines increased in intensity, indicating a re-illuminated BLR.

Spectropolarimetric data suggest intrinsic changes rather than obscuration.

Abstract

We present the first photo- and spectropolarimetric observations of the changing-look active galactic nucleus NGC~1346 taken with the Perkins telescope (November 2022, March 2025) and VLT/FORS2 (August and September 2024), which have revealed a new spectral transition. We find that NGC~1346 has reverted to its former (2001) type-1 spectral state after having been a type-2 active galactic nucleus from 2004 to 2022. More and more intense broad H$\beta$ (3720 $\pm$ 310 km.s$^{-1}$ full width at half maximum) and H$\alpha$ (2985 $\pm$ 230 km.s$^{-1}$) emission lines appeared between August and September 2024, suggesting that the broad-line region (BLR) is increasingly irradiated. The strong blueshift of the Balmer lines ($\sim$ 1600 km.s$^{-1}$) indicates that the gas is moving toward the observer, possibly due to a radiation-driven outflow. The strong difference between 2022's ($\sim$ 3\%) and 2024-2025's de-biased polarization ($\le$ 0.5\%), the blueing of the spectra between August and September 2024, and the slow rise in the integrated flux between 2022 and 2025 argue against asymmetric or temporary obscuration by a cloud passing in front of the line of sight. Two options, either a re-illumination of the BLR or a tidal disruption event, can explain the observed properties of NGC~1346. Timely follow-up observations taken while the phenomenon is still ongoing are needed to determine which of the two solutions is the correct one.