An elliptical accretion disk following the tidal disruption event AT 2020zso

¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
e-mail: twevers@eso.org
² Birmingham Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
³ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, USA
⁴ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
⁵ Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁶ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁷ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
⁸ CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Canada
⁹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
¹⁰ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
¹¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
¹² School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
¹³ Finnish Centre for Astronomy with ESO (FINCA), 20014 University of Turku, Finland
¹⁴ Tuorla Observatory, Department of Physics and Astronomy, 20014 University of Turku, Finland
¹⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill EH9 3HJ, UK
¹⁶ Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK
¹⁷ School of Physics and Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
¹⁸ Instituto di Astrofisica e Planetologia Spaziali (INAF), Via Fosso del Cavaliere 100, Roma 00133, Italy
¹⁹ The Oskar Klein Centre, Physics Department of Physics, Stockholm University, Albanova University Center, SE 106 91 Stockholm, Sweden
²⁰ Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA
²¹ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
²² Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
²³ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
²⁴ Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN, UK

Received: 8 November 2021
Accepted: 13 June 2022

Abstract

Aims. The modelling of spectroscopic observations of tidal disruption events (TDEs) to date suggests that the newly formed accretion disks are mostly quasi-circular. In this work we study the transient event AT 2020zso, hosted by an active galactic nucleus (AGN; as inferred from narrow emission line diagnostics), with the aim of characterising the properties of its newly formed accretion flow.

Methods. We classify AT 2020zso as a TDE based on the blackbody evolution inferred from UV/optical photometric observations and spectral line content and evolution. We identify transient, double-peaked Bowen (N III), He I, He II, and Hα emission lines. We model medium-resolution optical spectroscopy of the He II (after careful de-blending of the N III contribution) and Hα lines during the rise, peak, and early decline of the light curve using relativistic, elliptical accretion disk models.

Results. We find that the spectral evolution before the peak can be explained by optical depth effects consistent with an outflowing, optically thick Eddington envelope. Around the peak, the envelope reaches its maximum extent (approximately 10¹⁵ cm, or ∼3000–6000 gravitational radii for an inferred black hole mass of 5−10 × 10⁵ M_⊙) and becomes optically thin. The Hα and He II emission lines at and after the peak can be reproduced with a highly inclined (i = 85 ± 5 degrees), highly elliptical (e = 0.97 ± 0.01), and relatively compact (R_in = several 100 R_g and R_out = several 1000 R_g) accretion disk.

Conclusions. Overall, the line profiles suggest a highly elliptical geometry for the new accretion flow, consistent with theoretical expectations of newly formed TDE disks. We quantitatively confirm, for the first time, the high inclination nature of a Bowen (and X-ray dim) TDE, consistent with the unification picture of TDEs, where the inclination largely determines the observational appearance. Rapid line profile variations rule out the binary supermassive black hole hypothesis as the origin of the eccentricity; these results thus provide a direct link between a TDE in an AGN and the eccentric accretion disk. We illustrate for the first time how optical spectroscopy can be used to constrain the black hole spin, through (the lack of) disk precession signatures (changes in inferred inclination). We constrain the disk alignment timescale to > 15 days in AT2020zso, which rules out high black hole spin values (a < 0.8) for M_BH ∼ 10⁶ M_⊙ and disk viscosity α ≳ 0.1.