On the Missing Energy Puzzle of Tidal Disruption Events

For the majority of the tidal disruption event (TDE) candidates, the observed energy in the optical/near-UV bands is of order 10^{51} erg. We show that this observed energy is smaller than the minimum bolometric energy for the radiative inefficient accretion flow model by a factor of 10-100. We argue that this discrepancy is because the majority of the energy released is in the extreme-UV (EUV) band and/or in the form of relativistic jets beamed away from the Earth. The EUV scenario is supported by existing mid-infrared data and should be further tested by future dust reverberation observations. The jet scenario is disfavored by the radio observations of ASASSN-14li but may still be viable for other TDE candidates. We also provide evidence that, at least for some TDEs, most of the missing energy (in the EUV and/or in the form of jets) is released within a few times the orbital period of the most tightly bound material, which means (1) the circularization of the fallback stream may occur rapidly, and (2) the luminosity of the accretion flow or the jet power may not be capped near the Eddington level when the fallback rate is super-Eddington. For most other TDEs, this energy-release timescale is currently not strongly constrained.

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