A detailed spectroscopic study of Tidal Disruption Events

Abstract

Spectroscopically, TDEs are characterized by broad ( 10${}^4$ km/s) emission lines and show large diversity as well as different line profiles. After carefully and consistently performing a series of data reduction tasks including host galaxy light subtraction, we present here the first detailed, spectroscopic population study of 16 optical/UV TDEs. We report a time lag between the peaks of the optical light-curves and the peak luminosity of H$\alpha$ spanning between 7 - 45 days. If interpreted as light-echoes, these lags correspond to distances of 2 - 12 x 10${}^{16}$ cm, one to two orders of magnitudes larger than the estimated blackbody radii (R${}_{\mathrm{BB}}$) of the same TDEs and we discuss the possible origin of this surprisingly large discrepancy. We also report time lags for the peak luminosity of He I $\lambda$5876 line; smaller than the ones of H$\alpha$ for H TDEs and similar or larger for N III Bowen TDEs. We report that N III Bowen TDEs have lower H$\alpha$ velocity widths compared to the rest of the TDEs in our sample and we also find that a strong X-ray to optical ratio might imply weakening of the line widths. Furthermore, we study the evolution of line luminosities and ratios with respect to their radii (R${}_{\mathrm{BB}}$) and temperatures (T${}_{\mathrm{BB}}$). We find a linear relationship between H$\alpha$ luminosity and the R${}_{\mathrm{BB}}$ and potentially an inverse power-law relation with T${}_{\mathrm{BB}}$ leading to weaker H$\alpha$ emission for T${}_{\mathrm{BB}}$ $\ge$ 25000 K. The He II/He I ratio becomes large at the same temperatures possibly pointing to an ionization effect. The He II/H$\alpha$ ratio becomes larger as the photospheric radius recedes, implying a stratified photosphere where Helium lies deeper than Hydrogen. We suggest that the large diversity of the spectroscopic features seen in TDEs along with their X-ray properties, can potentially be attributed to viewing angle effects.

Published in A&A (part of 2022 A&A Highlights). 31 pages (10 of the Appendix), 21 Figures

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