Polarimetric diversity in tidal disruption events: Comparative study of low-polarised sources with AT2020mot

¹ Institute of Astrophysics, FORTH, N.Plastira 100, Vassilika Vouton, 70013 Heraklion, Greece
² Department of Physics University of Crete, Voutes University Campus, 70013 Heraklion, Greece
³ National Institute for Astrophysics (INAF), Astronomical Observatory of Padova, IT-35122 Padova, Italy
⁴ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁵ Institutt for Fysikk, Norwegian University of Science and Technology, Høgskloreringen 5, Trondheim 7491, Norway
⁶ Department of Physics and Astronomy, 20014 University of Turku, Turku, Finland
⁷ Finnish Centre for Astronomy with ESO (FINCA), Quantum, Vesilinnantie 5, 20014 University of Turku, Turku, Finland
⁸ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁹ Nordic Optical Telescope, Rambla José Ana Fernández, Pérez 7, E-38711 Breña Baja, Spain
¹⁰ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹¹ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
¹² Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA
¹³ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
¹⁴ Leiden Observatory, Leiden University, PO Box 9513 2300 RA, Leiden, The Netherlands

^⋆ Corresponding author: afloris@ia.forth.gr

Received: 22 May 2025
Accepted: 5 October 2025

Abstract

Context. Tidal disruption events (TDEs) occur when a star is disrupted by the tidal forces of a supermassive black hole (SMBH), which produces bright multi-wavelength flares. Among these events, AT2020mot has so far exhibited the highest recorded optical polarisation, with tidal shocks proposed as the primary source of its polarised emission.

Aims. We present a comprehensive analysis of 13 TDEs with available polarimetric observations, aiming to determine whether the unusually high polarisation of AT2020mot stems from unique physical processes or arises from mechanisms shared by other TDEs.

Methods. We present new optical polarisation measurements of TDEs obtained from multiple ground-based telescopes, combining them with optical, UV, and X-ray light curves from the Zwicky Transient Facility and the Swift observatory. We derived intrinsic TDE properties – such as SMBH and stellar masses – using MOSFiT and TDEMass, and compared them with those of the sample population.

Results. Our population study reveals that AT2020mot aligns with the broader TDE sample in terms of most physical properties, including blackbody temperature, luminosity, and rise timescales. However, its optical polarisation degree is exceptionally high compared to the low or undetected polarisation observed in other events. Additionally, according to our MOSFiT fit, AT2020mot has an elevated column density, which suggests a more complex environment than is typically assumed.

Conclusions. We conclude that although AT2020mot fits well within the general TDE population in terms of global characteristics, its extraordinarily high polarisation and higher column density challenge current models based purely on shock or reprocessing mechanisms. More extensive, time-resolved polarimetric monitoring of newly discovered TDEs will be critical to determine whether AT2020mot represents an outlier or the extreme end of a continuum of TDE properties.