Revisiting the unification of tidal disruption events with polarimetry

¹ DTU Space, Technical University of Denmark Elektrovej 327/328 DK-2800 Kongens Lyngby, Denmark
² Department of Physics, University of Warwick Gibbet Hill Road Coventry CV4 7AL, UK
³ Gemini Observatory / NSF’s NOIRLab Casilla 603 La Serena, Chile
⁴ Department of Physics and Astronomy, University of Turku FI-20014 Turku, Finland
⁵ Department of Physics and Earth Science, University of Ferrara Via Saragat 1 I-44122 Ferrara, Italy
⁶ INFN, Sezione di Ferrara Via Saragat 1 I-44122 Ferrara, Italy
⁷ INAF, Osservatorio Astronomico d’Abruzzo Via Mentore Maggini snc 64100 Teramo, Italy
⁸ Department of Physics, The University of Hong Kong Pokfulam Road Hong Kong, China
⁹ European Southern Observatory, Alonso de Córdova 3107 Casilla 19 Santiago, Chile
¹⁰ Astronomical Observatory, University of Warsaw Al. Ujazdowskie 4 00-478 Warszawa, Poland
¹¹ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC 08860 Castelldefels (Barcelona), Spain
¹² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n E-08193 Barcelona, Spain
¹³ School of Physics, Trinity College Dublin, The University of Dublin Dublin 2, Ireland
¹⁴ Instituto de Ciencias Exactas y Naturales (ICEN) Universidad Arturo Prat, Chile
¹⁵ Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast Belfast BT7 1NN, UK

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

Received: 4 July 2025
Accepted: 29 October 2025

Abstract

Aims. Tidal disruptions of stars by supermassive black holes produce emission at different wavelengths, but the optical emission is of ambiguous origin. A unification scenario of tidal disruption events (TDEs) has been proposed to explain the different classes of X-ray and optically selected events by introducing a dependence on the viewing angle and geometry. This work aims to test the unification scenario among optically bright TDEs using polarimetry.

Methods. By studying the optical linear polarisation of nineteen TDEs (nine of which are newly analysed in this work), we placed constraints on their photosphere geometry, inclination, and the emission process responsible for the optical radiation. We also investigated how these properties correlate with the relative X-ray brightness of the events, quantified by the L_X/L_g ratio.

Results. We find that 14 of the 16 non-relativistic events can be accommodated by the unification model. The continuum polarisation levels of the non-relativistic TDEs most often lie in the range P ∼ 1 − 2% (13 events), and for all except one event, they remain below 6%. For those optical TDEs that have multiple epochs of polarimetry, the continuum polarisation levels decrease with time after peak light for five of the ten events, increase for three events, and stay approximately constant for two events. When observed after +70 days (7/16 events), they become consistent with zero polarisation within uncertainties (5/7 events). This implies that the photosphere geometries of TDEs are at least initially asymmetric and evolve rapidly, which if tracing the formation of the accretion disc, suggests efficient circularisation. The polarisation signatures of emission lines of seven TDEs directly support a scenario in which optical light is reprocessed in an electron-scattering photosphere. TDEs are most often weak in X-rays when significantly polarised. However, a subset of events deviates from the unification model to some extent, suggesting this model may not fully capture the diverse behaviour of TDEs. Multi-epoch polarimetry plays a key role in understanding the evolution and emission mechanisms of TDEs.