An Energy Inventory of Tidal Disruption Events

Abstract

Tidal disruption events (TDEs) offer a unique opportunity to study a single super-massive black hole (SMBH) under feeding conditions that change over timescales of days or months. However, the primary mechanism for generating luminosity during the flares remains debated. Despite the increasing number of observed TDEs, it is unclear whether most of the energy in the initial flare comes from accretion near the gravitational radius or from circularizing debris at larger distances from the SMBH. The energy dissipation efficiency increases with decreasing radii, therefore by measuring the total energy emitted and estimating the efficiency we can derive clues about the nature of the emission mechanism. Here we calculate the integrated energy, emission timescales, and average efficiencies for the TDEs using the Modular Open Source Fitter for Transients ({\tt MOSFiT}). Our calculations of the total energy generally yield higher values than previous estimates. This is predominantly because, if the luminosity follows the mass fallback rate, TDEs release a significant fraction of their energy long after their light curve peaks. We use {\tt MOSFiT} to calculate the conversion efficiency from mass to radiated energy, and find that for many of the events it is similar to efficiencies inferred for active galactic nuclei. There are, however, large systematic uncertainties in the measured efficiency due to model degeneracies between the efficiency and the mass of the disrupted star, and these must be reduced before we can definitively resolve the emission mechanism of individual TDEs.