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 H α emission lines. We model medium-resolution optical spectroscopy of the He II (after careful de-blending of the N III contribution) and H α 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 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 H α 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 ∼ 10 6 M ⊙ and disk viscosity α ≳ 0.1.
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This content will become publicly available on January 24, 2025
Panic at the ISCO: Time-varying Double-peaked Broad Lines from Evolving Accretion Disks Are Common among Optically Variable AGNs
About 3%–10% of Type I active galactic nuclei (AGNs) have double-peaked broad Balmer lines in their optical spectra originating from the motion of gas in their accretion disk. Double-peaked profiles arise not only in AGNs, but occasionally appear during optical flares from tidal disruption events and changing-state AGNs. In this paper, we identify 250 double-peaked emitters (DPEs) among a parent sample of optically variable broad-line AGNs in the Zwicky Transient Facility (ZTF) survey, corresponding to a DPE fraction of 19%. We model spectra of the broad H
- Award ID(s):
- 2108402
- NSF-PAR ID:
- 10524207
- Publisher / Repository:
- AAS
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 961
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 172
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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