We present late-time optical spectroscopy taken with the Large Binocular Telescope ’ s Multi-Object Double Spectrograph , late-time Swift UVOT and XRT observations , as well as improved ASAS-SN pre-discovery limits on the nearby ( d = 90.3 Mpc , z = 0.0206 ) tidal disruption event ( TDE ) ASASSN-14li .
The late-time optical spectra show H \alpha emission well in excess of that seen in the SDSS host galaxy spectrum , indicating that the processes powering the luminous flares associated with TDEs can operate for several hundreds of days .
The Swift observations reveal the presence of lingering apparently thermal UV ( T _ { UV } \sim 3.5 \times 10 ^ { 4 } K ) and X-ray ( T _ { X } \sim 7 \times 10 ^ { 5 } K ) emission .
The characteristic temperatures evolve by , at most , a factor of \sim 2 over the 600 day follow-up campaign .
The X-ray , UV , and H \alpha luminosities evolve roughly in tandem and at a rate that is consistent with a power-law decay at late times .
This behavior is in stark contrast with the majority of optically discovered TDEs , which are X-ray faint and evolve on shorter timescales .
Finally we address how the unique properties of ASASSN-14li can be used to probe the relationship between the TDE rate and host galaxy properties .