The Spectral Evolution of at 2018dyb and the Presence of Metal Lines in Tidal Disruption Events

Giorgos Leloudas, Lixin Dai, Iair Arcavi, Paul M. Vreeswijk, Brenna Mockler, Rupak Roy, Daniele B. Malesani, Steve Schulze, Thomas Wevers, Morgan Fraser, Enrico Ramirez-Ruiz, Katie Auchettl, Jamison Burke, Giacomo Cannizzaro, Panos Charalampopoulos, Ting Wan Chen, Aleksandar Cikota, Massimo Della Valle, Lluis Galbany, Mariusz GromadzkiKasper E. Heintz, Daichi Hiramatsu, Peter G. Jonker, Zuzanna Kostrzewa-Rutkowska, Kate Maguire, Ilya Mandel, Matt Nicholl, Francesca Onori, Nathaniel Roth, Stephen J. Smartt, Lukasz Wyrzykowski, Dave R. Young

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Abstract

We present light curves and spectra of the tidal disruption event (TDE) ASASSN-18pg/AT 2018dyb spanning a period of one year. The event shows a plethora of strong emission lines, including the Balmer series, He ii, He i, and metal lines of O iii λ3760 and N iii λλ4100, 4640 (blended with He ii). The latter lines are consistent with originating from the Bowen fluorescence mechanism. By analyzing literature spectra of past events, we conclude that these lines are common in TDEs. The spectral diversity of optical TDEs is thus larger than previously thought and includes N-rich events besides H- and He-rich events. We study how the spectral lines evolve with time, by means of their width, relative strength, and velocity offsets. The velocity width of the lines starts at ∼13,000 km s-1 and decreases with time. The ratio of He ii to N iii increases with time. The same is true for ASASSN-14li, which has a very similar spectrum to AT 2018dyb but its lines are narrower by a factor of >2. We estimate a black hole mass of M BH = 3.3-2.0 +5.0 × 106 Mo˙ by using the M-σ relation. This is consistent with the black hole mass derived using the MOSFiT transient fitting code. The detection of strong Bowen lines in the optical spectrum is an indirect proof for extreme ultraviolet and (reprocessed) X-ray radiation and favors an accretion origin for the TDE optical luminosity. A model where photons escape after multiple scatterings through a super-Eddington thick disk and its optically thick wind, viewed at an angle close to the disk plane, is consistent with the observations.

Original languageEnglish
Article number218
Number of pages15
JournalThe Astrophysical Journal
Volume887
Issue number2
DOIs
Publication statusPublished - 20 Dec 2019

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