Guillochon et al. (2014) suggest that the broad lines seen in TDEs are produced in a BLR structure (analogous to the one seen in AGNs) which lies above and below the forming accretion disk. Furthermore, they comment that if this is true, it would be reasonable to expect that these two astrophysical phenomena (i.e., AGNs and TDEs) should have many similarities in terms of velocity structures, as well as the components of the structure that play a key role in the production of the radiated light and the emergent emission lines. If their prediction is valid, it is not surprising that TDEs show the observed time lags pointed out in this work, as time lags (caused by the response of line luminosities to variations in the continuum) are commonly observed in AGNs. It has been suggested that the line emitting region must be stratified, where Helium is closer to the black hole than Hydrogen (Guillochon et al. 2014; Roth et al. 2016). Regular AGNs are well known to be stratified as well with He II, He I, Hβ and Hα being emitted from closest to furthest from the black hole (e.g., see Clavel et al. 1991; Peterson & Wandel 1999). To test this, we investigated how lagged the Helium lines are compared to Hydrogen (see Sects. 5.1.1 and 5.1.2). We chose He I 5876 Å as this line is more isolated and easier to measure than He II. We find a difference, depending on the spectroscopic type: H TDEs show larger lags in Hα than He I. On the other hand, for N III Bowen TDEs, He I has similar lag values to Hα. The average (lower-limit) lag value for Hα is 40 days for the H TDEs and 12 days for the N III Bowen TDEs (without including ASASSN-15oi because it does not belong in either of these categories as it is a He TDE with weak Balmer lines in some epochs). The average (lower-limit) lag value for He I is 15.48 light days for N III Bowen TDEs. For the H TDEs, only AT2018zr shows a clear lag in He I since for ASASSN-14ae and LSQ12dyw we can only place upper-limits). These results are visualized in Fig. 18 where we plot the time lags against the blackbody radii values at peak for each TDE. The Hα lags (filled symbols) increase with increasing RBB (the dashed line is a fit to Hα and the dot-dashed one is the same without fitting the three lower-limit points). On the other hand, He I (empty symbols) does not show such a correlation and the lags remain small (mostly upper-limits) even for the larger radii. This picture can be understood by a combination of two factors: (i) N III Bowen TDEs have smaller radii than H TDEs (van Velzen et al. 2021; ii) Helium lies deeper in the photosphere than Hydrogen (Roth et al. 2016). The ionization energy of neutral Hydrogen is 13.6 eV while the one of neutral Helium is 24.6 eV hence Helium would need a larger temperature in order to get ionized, hinting that it is indeed lying closer to the black hole than Hydrogen – consistent with a stratified photosphere. We find here, however, that these differences are minimized in the more compact Bowen TDEs. Guillochon et al. (2014) focus their study on PS1-10jh, the only TDE in our sample that shows no Hydrogen at all, but only strong He II. They suggest that the absence of Hydrogen lines indicates that the accretion disk had not yet extended to the distances required to produce the Hydrogen lines. Their calculations imply that He II 4686 Å should be produced at a distance of ∼2.1 light days from the SMBH (see their Fig. 7). Such small lags are beyond the precision that can be attained with the present data, as daily spectroscopic observations would be required.
Citation: Roth et al. 2016; Roth et al. 2016
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