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

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 11 rue de l’université, 67000 Strasbourg, France
² Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19c, B5c, 4000 Liège, Belgium
³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA

^⋆ Corresponding author: frederic.marin@astro.unistra.fr

Received: 16 April 2025
Accepted: 6 July 2025

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 Hβ (3720 ± 310 km.s⁻¹ full width at half maximum) and Hα (2985 ± 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 (∼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 (∼3%) and 2024–2025’s de-biased polarization (≤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.