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

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 λλ 4100, 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 Hand 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 +5.0 −2.0 × 10 6 M 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.

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

“…We also plot the Xray light curve to highlight the X-ray to optical ratio, which is 10 −2.8 before peak. Since the X-rays appear to rise after the optical emission starts to fade, this ratio increases to ≈10 −2.0 − 10 −1.8 between 20-50 d after bolometric peak, and reaches ≈10 −1.4 beyond 50 d. (Gezari et al 2012;Arcavi et al 2014;Chornock et al 2014;Holoien et al 2014Holoien et al , 2016aBlagorodnova et al 2017;Leloudas et al 2019;Nicholl et al 2019b;van Velzen et al 2019;Gomez et al 2020) We plot the bolometric light curve compared to other wellobserved TDEs in Fig. 10.…”

“…They are classified according to their spectra, generally exhibiting broad, low equivalent width 1 emission lines of hydrogen, neutral and ionized helium, and Bowen fluorescence lines of doubly ionized nitrogen and oxygen (e.g. Arcavi et al 2014;Leloudas et al 2019). This prompted van to suggest three subclasses labelled TDE-H, TDE-He, and TDE-Bowen, though some TDEs defy a consistent classification by changing their apparent spectral type as they evolve (Nicholl et al 2019b).…”

“…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.…”

First light from tidal disruption events

“…LSST (Bricman & Gomboc 2019). In particular, multiple well-sampled observations in the optical and UV bands have now managed to catch the early rise to the peak of the lightcurve, which probes the first interactions experienced by the debris as they fall back near the black hole (Gezari et al 2012;Chornock et al 2014;Arcavi et al 2014;Blagorodnova et al 2017;Leloudas et al 2019;Holoien et al 2019a,b;Nicholl et al 2019;van Velzen et al 2019;Holoien et al 2020;Nicholl et al 2020). This phase is often associated with signs of outflow evidenced by the detection of radio emission and blue-shifted atomic lines (e.g.…”

“…Strong Bowen fluorescence lines have also been detected (e.g. Blagorodnova et al 2019;Leloudas et al 2019) whose formation necessitates the presence of extreme-ultraviolet radiation likely produced by prompt gas accretion. This scenario is additionally supported in some events by the early detection of a doublepeaked emission line (Hung et al 2020;Short et al 2020), potentially originating from a nearly circular accretion disc.…”