Probing the UV spectral evolution of a unique TDE with early bright X-ray emission

We received approval to use the Hubble Space Telescope for four orbits to conduct late-time UV spectral observations of a Tidal Disruption Event, AT2020-afhd, aimed at testing my analytical model predictions.

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Abstract: Stellar tidal disruption events (TDEs) by supermassive black holes (SMBHs) are nowadays predominantly discovered by optical time-domain surveys. However, the exact physical origin of their optical emission, whether produced in the accretion phase or in the stream-stream collision phase, remains highly controversial, thereby becoming the greatest obstacle to the scientific applications of TDEs. We report the discovery of an intriguing TDE with an early bright X-ray emission, indicating a rapid onset of accretion. An ultraviolet spectrum observed promptly around the optical peak revealed a remarkably steep continuum slope, with a notable blue excess compared to the standard blackbody fit to the photometric spectral energy distribution (SED). However, the overall SED is in excellent agreement with the prediction of the reprocessing model, giving a bolometric luminosity at least an order of magnitude higher than the conventional measurement. It is shown directly for the first time that a naive blackbody assumption of the SED of optical TDEs has seriously underestimated their true intrinsic luminosity, leading to the missing energy puzzle. Timely ultraviolet spectroscopic observations of more X-ray bright optical TDEs will help to further explore the intrinsic SEDs and their associated physics.