Revisiting the unification of tidal disruption events with polarimetry
: Wichern, H. C. I.; Leloudas, G.; Pursiainen, M.; Cikota, A.; Jaisawal, G. K.; Charalampopoulos, P.; Bulla, M.; Dai, L.; Anderson, J. P.; Gromadzki, M.; Gutierrez, C. P.; Muller-Bravo, T. E.; Nicholl, M.
Publisher: EDP Sciences
: 2026
Astronomy and Astrophysics
: A250
: 705
: 0004-6361
: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202556270
: https://doi.org/10.1051/0004-6361/202556270
: https://research.utu.fi/converis/portal/detail/Publication/515556472
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 LX/Lg 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 similar to 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.
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We would like to thank the anonymous referee for providing helpful suggestions that improved our manuscript. We thank Kohki Uno for sharing data and polarimetry values for AT 2023clx and Klaas Wiersema for checking the polarimetry of OGLE16aaa. GL was supported by a research grant (VIL60862) from VILLUM FONDEN. MP acknowledges support from a UK Research and Innovation Fellowship (MR/T020784/1). MB acknowledges the Department of Physics and Earth Science of the University of Ferrara for the financial support through the FIRD 2024 grant. LD acknowledges the support from the National Natural Science Foundation of China and the Hong Kong Research Grants Council (N_HKU782/23, HKU 17314822, 17305124). TEMB is funded by Horizon Europe ERC grant no. 101125877. CPG acknowledges financial support from the Secretary of Universities and Research (Government of Catalonia) and by the Horizon 2020 Research and Innovation Programme of the European Union under the Marie Sk & lstrok;odowska-Curie and the Beatriu de Pinos 2021 BP 00168 programme, from the Spanish Ministerio de Ciencia e Innovacion (MCIN) and the Agencia Estatal de Investigacion (AEI) 10.13039/501100011033 under the PID2023-151307NB-I00 SNNEXT project, from Centro Superior de Investigaciones Cientificas (CSIC) under the PIE project 20215AT016 and the program Unidad de Excelencia Maria de Maeztu CEX2020-001058-M, and from the Departament de Recerca i Universitats de la Generalitat de Catalunya through the 2021-SGR-01270 grant. MN is supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 948381). Based on observations collected at the European Southern Observatory under ESO programme(s) 199.D-0143(R), 0103.D-0350(A), 106.214S.001, 106.216C.008, 106.216C.009, 106.216C.011, 108.222Q.001, 109.23FR.001, 111.24ME.001, 112.25JQ.001, 112.25JQ.005, and 112.25JQ.008. Based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The data presented here were obtained with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOT. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham). This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile. This research has made use of the SVO Filter Profile Service (http://svo2.cab.inta-csic. es/theory/fps/) supported from the Spanish MINECO through grant AYA2017-84089. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester, and data obtained through the High Energy Astrophysics Science Archive Research Center online service, provided by the NASA/Goddard Space Flight Center. This research has made use of data obtained from the Chandra Data Archive provided by the Chandra X-ray Center (CXC). Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA This publication makes use of data products from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which is a joint project of the Jet Propulsion Laboratory/California Institute of Technology and the University of Arizona. NEOWISE is funded by the National Aeronautics and Space Administration. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
