Optical/UV Emission in the Tidal Disruption Event ASASSN-14li: Implications of Disc Modeling

Abstract

We predict late-time optical/UV emission from tidal disruption events (TDEs) from our slim accretion disc model \citep{Wen20} and explore the impact of the black hole mass $M_{\bullet }$, black hole spin $a_{\bullet }$, and accretion disc size. We use these synthetic spectra to successfully fit the multi-band \emph{Swift} observations of ASASSN-14li at >350 days, setting only the host galaxy extinction and outer disc radius as free parameters and employing the $M_{\bullet }$, $a_{\bullet }$, 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 \emph{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 $\approx$2$\times$ the tidal radius and $\sim$constant at late times, showing that viscous spreading is slow or non-existent; (4) the shock model can be self-consistent if $M_{\bullet }\lesssim 10^{6.75}$M${}_{\odot }$, i.e., on the low end of ASASSN-14li's $M_{\bullet }$ range ($10^{6.5-7.1}$M${}_{\odot }$; 1$\sigma$ 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${}_{\odot }$, consistent with a (plausible) disruption of a solar-mass star.

arXiv comment15 pages, 10 figures. Accepted for publication in MNRAS

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