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 ASAS-SN 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 days. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disk emission, and back to smooth at late times. The Balmer lines are unlike typical AGN in that they show a flat Balmer decrement (Hα/Hβ ∼ 1.5), suggesting the lines are collisionally excited rather than being produced via photo-ionisation. The flat Balmer decrement together with the complex profiles suggest that the emission lines originate in a disk 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.

An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz

“…This is consistent with the size of TDE accretion discs in the simulations of . Disc profiles in TDE emission lines have been claimed in PT09djl (Arcavi et al 2014;Liu et al 2017) and AT2018hyz (Hung et al 2020;Short et al 2020). However the difficulty in interpreting these line profiles is illustrated by the case of AT2018zr (also called PS18kh), which had flat-topped Balmer lines argued by Holoien et al (2019a) to originate in an elliptical disc and by Hung et al (2019) to instead come from an outflow.…”

“…On the other hand the transient appearance of a double-peaked profile at this phase after peak would be reminiscent of AT2018hyz. The transience of these signatures could be due to either optical depth effects (Gomez et al 2020;Short et al 2020) or contamination by an additional emission component (Hung et al 2020). Detailed time-series modelling with disc profiles will be needed to confirm if this scenario is compatible with AT2019qiz.…”

“…It has recently been pointed out that a number of TDEs show much flatter Balmer decrements (H α/H β ≈ 1; Leloudas et al 2019;Short et al 2020), which may indicate collisional excitation of these lines, e.g. in a disc chromosphere (Short et al 2020).…”

“…This ratio can be difficult to measure in TDEs due to line blending, but the excellent data quality available for AT2019qiz confirms a higher Balmer ratio in this event. If we assume a collisional origin for the Balmer lines in AT2019qiz, the implied temperature of the line-forming region is 3000 K, following Short et al (2020). Alternatively, a large Balmer Figure 18.…”

“…Several lines of evidence have indicated that accretion discs do form promptly even in X-ray faint TDEs: Bowen fluorescence lines that require excitation from far-UV photons (Blagorodnova et al 2019;Leloudas et al 2019); low-ionization iron emission appearing shortly after maximum light (Wevers et al 2019b); and recently the direct detection of double-peaked Balmer lines that match predicted disc profiles (Hung et al 2020;Short et al 2020). Thus a critical question is to understand the nature and origin of the implied reprocessing layer.…”