Elliptical Accretion and Low Luminosity from High Accretion Rate Stellar Tidal Disruption Events

Models for tidal disruption events (TDEs) in which a supermassive black hole disrupts a star commonly assume that the highly eccentric streams of bound stellar debris promptly form a circular accretion disk at the pericenter scale. However, recent numerical simulations (Shiokawa et al., 2015) demonstrated that dissipation via hydrodynamical shocks is insufficient to circularize debris, and the flow retains its initial semi-major axis scale throughout the first ~10 orbits of the event. The bolometric peak luminosity of most TDE candidates, a few x 10^{44} erg/s, implies that we observe only ~1% of the energy expected from radiatively efficient accretion. Motivated by these results, (Piran et al., 2015) suggested that the observed optical TDE emission is powered by shocks at the apocenter between freshly infalling material and earlier arriving matter. This model explains the small radiated energy, the low temperature, and the large radius implied by the observations as well as the t^{-5/3} light curve. However the question of the system's low {bolometric} efficiency remains unanswered. We suggest that the high orbital energy and low angular momentum of the flow make it possible for magnetic stresses to reduce the matter's already small angular momentum to the point at which it can fall ballistically into the SMBH before circularization. As a result, the efficiency is only ~1--10% of a standard accretion disk's efficiency. Thus, the intrinsically high eccentricity of the tidal debris naturally explains why most TDE candidates are fainter than expected.