The tidal disruption event AT 2018hyz - I : double-peaked emission lines and a flat Balmer decrement
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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.
Short , P , Nicholl , M , Lawrence , A , Gomez , S , Arcavi , I , Wevers , T , Leloudas , G , Schulze , S , Anderson , J P , Berger , E , Blanchard , P K , Burke , J , Castro Segura , N , Charalampopoulos , P , Chornock , R , Galbany , L , Gromadzki , M , Herzog , L J , Hiramatsu , D , Horne , K , Hosseinzadeh , G , Howell , D A , Ihanec , N , Inserra , C , Kankare , E , Maguire , K , McCully , C , Müller Bravo , T E , Onori , F , Sollerman , J & Young , D R 2020 , ' The tidal disruption event AT 2018hyz - I : double-peaked emission lines and a flat Balmer decrement ' , Monthly Notices of the Royal Astronomical Society , vol. 498 , no. 3 , pp. 4119–4133 . https://doi.org/10.1093/mnras/staa2065
Monthly Notices of the Royal Astronomical Society
Copyright © 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the final published version of the work, which was originally published at https://doi.org/10.1093/mnras/staa2065.
DescriptionPublished in MNRAS. Accompanied by companion paper Gomez et al. (2020). Funding: KH acknowledges support from STFC grant ST/R000824/1.
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