Optical/UV emission in the Tidal Disruption Event ASASSN-14li: implications of disc modelling

We predict late-time optical/UV emission from tidal disruption events (TDEs) from our slim accretion disc model (Wen et al. 2020) and explore the impact of the black hole mass M_•, black hole spin a_•, and accretion disc size. We use these synthetic spectra to successfully fit the multiband Swift observations of ASASSN-14li at >350 d, setting only the host galaxy extinction and outer disc radius as free parameters and employing the M_•, a_•, disc inclination, and disc accretion rates derived from fitting 10 epochs of ASASSN-14li's X-ray spectra with the slim disc. To address the nature of the early-time optical/UV emission, we consider two models: shock dissipation and reprocessing. We find that (1) the predicted late-time optical/UV colour (e.g. u - w2) is insensitive to black hole and disc parameters unless the disc spreads quickly; (2) a starburst galaxy extinction model is required to fit the data, consistent with ASASSN-14li's post-starburst host; (3) surprisingly, the outer disc radius is ≈2 × the tidal radius and ~constant at late times, showing that viscous spreading is slow or non-existent; (4) the shock model can be self-consistent if M_• ≲ 10^6.75 M_⊙, i.e. on the low end of ASASSN-14li's M_• range (10^6.5-7.1 M_⊙; 1σ CL); larger black hole masses require disruption of an unrealistically massive progenitor star; (5) the gas mass needed for reprocessing, whether by a quasi-static or an outflowing layer, can be <0.5 M_⊙, consistent with a (plausible) disruption of a solar-mass star.