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

Abstract

Aims. Modeling 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 deblending 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 peak can be explained by optical depth effects consistent with an outflowing, optically thick Eddington envelope. Around peak the envelope reaches its maximum extent (approximately 10 15 cm, or ∼ 3000 -6000 gravitational radii for an inferred black hole mass of 5 − 10 × 10 5 M ) and becomes optically thin. The Hα and He ii emission lines at and after 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 SMBH 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 ∼ 10 6 M and disk viscosity α 0.1.

The Final Season Reimagined: 30 Tidal Disruption Events from the ZTF-I Survey

“…In particular, double-peaked emission lines, which are seen in some AGN, are thought to originate from the outer regions of an inclined accretion disk. Wevers et al (2022) examined the line profiles of AT2020zso, a TDE we have included in our sample, and find that the emission lines after peak can be reproduced with a highly inclined, highly elliptical, and relatively compact accretion disk, further supporting the unification picture where viewing angle determines the observed properties of a TDE. In Figure 12, we show our spectrum of AT2020zso along with several other extreme broad and flat-topped/double-peaked TDEs in our sample.…”