Optical polarisation of stellar-fed active and quiescent supermassive black holes

¹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Universitätsstraße 150, 44801 Bochum, Germany
² Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
³ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen, Denmark
⁴ European Space Agency, European Space Astronomy Centre, E-28692 Villanueva de la Cañada, Madrid, Spain
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶ Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
⁷ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA

^★ Corresponding author: jordana@astro.ruhr-uni-bochum.de

Received: 8 July 2025
Accepted: 1 October 2025

Abstract

Context. The advent of wide-field optical surveys with multi-wavelength capabilities has been a breakthrough in the detection and characterisation of bright, long-lasting optical flares from supermassive black holes, such as tidal disruption events (TDEs), Bowen flares in active galactic nuclei (AGNs), and changing-look AGNs. Yet, the physical mechanisms powering these diverse transient events remain highly debated.

Aims. We aim to provide novel model constraints by utilising broadband optical polarimetry to study five TDEs. Our goal is to enhance the understanding of the reprocessing material involved in TDEs and to compare our findings with similar flaring activity observed in AGNs.

Methods. Using the MOPTOP polarimeter on the 2 metre Liverpool Telescope, we conducted a monitoring campaign targeting three optically discovered TDEs (AT2024bgz, AT2024pvu, and AT2024wsd) and two Bowen flares in AGNs (AT2020afhd and AT2019aalc).

Results. The three thermal TDEs show low intrinsic polarisation levels (ΔP ≈ 0 − 6%) with stable polarisation angles. The Bowen flares also show variable polarisation degrees (ΔP ≈ 0 − 8%) but significant polarisation angle variability: AT2020afhd exhibited a Δθ = 83 ± 8° shift at 150 days post-optical peak, while the AT2019aalc displayed quasi-periodic swings of Δθ ≈ 40° amplitude starting 190 days after peak brightness.

Conclusions. The TDEs of this study are well described by models invoking rapid disk formation and reprocessed emission from optically thick outflows, whereas the Bowen flares reveal more complex reprocessing geometries, potentially consistent with TDEs occurring in AGN gas-rich environments. We find that moderate polarisation is observed at later times for TDEs with low-Eddington ratios and highly extended photospheres. This implies that, as the accretion level declines, we expect more asymmetric reprocessing layers along a given viewing angle. Since the outflow density and velocity depend sensitively on the inclination angle, we expect TDEs with low-Eddington ratios and highly extended photospheres to exhibit varying levels of polarisation. The polarisation of AT2019aalc (Seyfert 1) hints at a clumpy, asymmetric outflow and the presence of a tilted, precessing accretion disk, while the polarisation of AT2020afhd (AGN type 2) is consistent with the detection of a scattered light echo.