Evidence for rapid disc formation and reprocessing in the X-ray bright tidal disruption event candidate AT 2018fyk

Abstract

We present optical spectroscopic and Swift UVOT/XRT observations of the X-ray and UV/optical bright tidal disruption event (TDE) candidate AT 2018fyk/ASASSN-18ul discovered by ASAS-SN. The Swift lightcurve is atypical for a TDE, entering a plateau after ∼40 days of decline from peak. After 80 days the UV/optical lightcurve breaks again to decline further, while the X-ray emission becomes brighter and harder. In addition to broad H, He and potentially O/Fe lines, narrow emission lines emerge in the optical spectra during the plateau phase. We identify both high ionisation (O iii) and low ionisation (Fe ii) lines, which are visible for ∼45 days. We similarly identify Fe ii lines in optical spectra of ASASSN-15oi 330 d after discovery, indicating that a class of Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-line Seyfert 1 galaxies and some extreme coronal line emitters suggests that TDEs are capable of creating similar physical conditions in the nuclei of galaxies. The Fe ii lines can be associated with the formation of a compact accretion disk, as the emergence of low ionisation emission lines requires optically thick, high density gas. Taken together with the plateau in X-ray and UV/optical luminosity this indicates that emission from the central source is efficiently reprocessed into UV/optical wavelengths. Such a twocomponent lightcurve is very similar to that seen in the TDE candidate ASASSN-15lh, and is a natural consequence of a relativistic orbital pericenter.

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

“…The temperature remains approximately constant around ∼ 25 000 K. In that sense, AT 2018dyb is similar to many other optical TDEs (e.g. Holoien et al 2016b;Hung et al 2017) although it is now documented that the temperature evolution of TDEs is quite diverse (Holoien et al 2016a(Holoien et al , 2019Leloudas et al 2016;Wevers et al 2019b).…”

To TDE or not to TDE: the luminous transient ASASSN-18jd with TDE-like and AGN-like qualities

“…Multi-epoch SALT spectra of the emission features near 4600Å. Black lines denote He and potential N iii-v lines present in the spectra, whereas grey lines mark potential Fe ii lines that are observed in AGNs and some TDEs (Wevers et al 2019b). Fe ii lines are taken from Kovačević et al (2010).…”

“…By comparison, most TDEs with well-sampled UV/optical light curves show only smooth, monotonic fading. Some recently discovered TDEs have shown deviations from monotonic fading: ASASSN-18ul showed a light curve plateau ∼40 d after peak brightness (Wevers et al 2019b) and ASASSN-19bt showed a short flare before reaching peak brightness (Holoien et al 2019b). In a more extreme example, PS18kh showed a UV re-brightening ∼50 d after peak (Holoien et al 2019a) and then, after a seasonal gap due Sun constraints, appeared to return to near-peak luminosity (van Velzen et al 2019a).…”

“…Its X-ray flux increased around ∼200 d before decaying ∼350 d after discovery, which is believed to have been caused by ei-ther inefficient circularization that resulted in delayed accretion, or material surround the TDE becoming optically thin a few months after discovery (Gezari et al 2017b;Holoien et al 2018). The optically-detected TDEs PS18kh (Holoien et al 2019a;van Velzen et al 2019a) and ASASSN-19bt (Holoien et al 2019b) have also shown X-ray emission, but only the TDE candidate ASASSN-18ul (Wevers et al 2019b) exhibits a X-ray light curve similar to that of ASASSN-18jd. ASASSN-18ul shows a relatively flat X-ray light curve with small timescale variations where the X-ray luminosity varies by more than an order of magnitude (∼10 42 −10 43.2 erg s −1 ).…”

Multiwavelength Study of an X-Ray Tidal Disruption Event Candidate in NGC 5092

“…Current models for the optical emission mainly include (1) thermal emission from a disk (Lodato & Rossi 2011); (2) emission during the formation of the accretion disk, in which the emission is powered by shocks from intersecting stellar debris streams (Piran et al 2015); and (3) reprocessing of the hotter disk emission by the outer envelope, where the envelope is formed by outflow during the accretion process (Guillochon et al 2014;Metzger & Stone 2016) or stellar debris (Loeb & Ulmer 1997). Only a few TDEs have contemporaneous X-ray and optical detections, e.g., ASASSN-14li (Miller et al 2015;Cenko et al 2016;Holoien et al 2016), XMMSL1 J074008.2-853927 (Saxton et al 2017), AT2018fyk (Wevers et al 2019), AT2018zr (van Velzen et al 2019, and ASASSN-19bt (Holoien et al 2019). A unified, viewing-angle-dependent model has been proposed by Dai et al (2018) for interpreting the various classes of observed TDEs.…”