The tidal disruption event AT 2018hyz - I. Double-peaked emission lines and a flat Balmer decrement
Abstract
We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the All-Sky Automated Survey for Supernova survey at an absolute magnitude of MV ~ -20.2 mag, in the nucleus of a quiescent galaxy with strong Balmer absorption lines. AT 2018hyz shows a blue spectral continuum and broad emission lines, consistent with previous TDE candidates. High cadence follow-up spectra show broad Balmer lines and He I in early spectra, with He II making an appearance after ~70-100 d. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disc emission, and back to smooth at late times. The Balmer lines are unlike typical active galactic nucleus in that they show a flat Balmer decrement (Hα/Hβ ~ 1.5), suggesting the lines are collisionally excited rather than being produced via photoionization. The flat Balmer decrement together with the complex profiles suggests that the emission lines originate in a disc chromosphere, analogous to those seen in cataclysmic variables. The low optical depth of material due to a possible partial disruption may be what allows us to observe these double-peaked, collisionally excited lines. The late appearance of He II may be due to an expanding photosphere or outflow, or late-time shocks in debris collisions.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2020
- DOI:
- arXiv:
- arXiv:2003.05470
- Bibcode:
- 2020MNRAS.498.4119S
- Keywords:
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- black hole physics;
- galaxies: active;
- galaxies: individual: AT 2018hyz;
- transients: tidal disruption events;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Published in MNRAS. Accompanied by companion paper Gomez et al. (2020)