Spectropolarimetry of five changing-look active galactic nuclei

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

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 November 2025
Accepted: 9 April 2026

Abstract

Context. A growing number of active galactic nuclei (AGNs) show dramatic changes in their spectral classification. The mechanisms behind the appearance and disappearance of the optical broad emission lines, as well as the brightening or fading of AGNs, are still elusive.

Aims. The goal of this study is to investigate this problem from a different angle by using the unique constraints that (spectro)polarimetry can bring.

Methods. We acquired polarized spectra of five “changing-look” AGNs (CLAGNs) using the VLT/FORS2 between July and September 2024, using the 300V grism and a 1″ width slit. The sample consists of IRAS 23226-3843, NGC 1346, NGC 2617, NGC 7582, and NGC 7603.

Results. Three of the five targets show newly re-emerged broad-line components associated with low-polarized, often wavelength-independent continua, attributed to electron scattering in an equatorial region seen close to pole-on. Their associated broad Hβ lines are systematically blueshifted, while their Hα profiles are predominantly double-peaked around the central emission wavelength, indicative of a kinematically disturbed or disk-dominated region responding to ionizing continuum changes. The remaining two objects do not show clear evidence of any newly emerged polarized broadline emission.

Conclusions. Our findings are consistent with accretion-driven changes as the dominant driver of the observed spectral evolution in CLAGNs, with polarimetric properties that are broadly compatible with this interpretation for four out of the five sources.