Evolution of Thick Accretion Disks Produced by Tidal Disruption Events

Abstract

Geometrically thick disks may form after tidal disruption events, and rapid accretion may lead to short flares followed by long-term, lower-level emission. Using a novel accretion disk code which relies primarily on global conservation laws and the assumption that viscosity is everywhere positive, a broad range of physically allowed evolutionary sequences of thick disks is investigated. The main result is that accretion in the thick disk phase can consume only a fraction of the initial disk material before the disk cools and becomes thin. This fraction is ~0.5-0.9 for disruptions around 10^6 to 10^7 M_\odot black holes and is sensitive to the mean angular momentum of the disk. The residual material will accrete in some form of thin disk over a longer period of time. The initial thick disk phase may reduce the dimming timescale of the disk by a factor of ~2 from estimates based on thin disks alone. Assuming an 0.5 M_\odot initial thick disk, even if the thin disks become advection dominated, the black hole mass to light ratio can rise above M_\odot/L_\odot = 1 in no less than 20 (0.1/α) to 2000 (0.1/α) years following a tidal disruption event, depending on the mass of the black hole and the initial conditions of the encounter. The long-term emission will be most prevalent around lower mass, 10^6 M_\odot black holes. If the tidal disruption rates in these galactic nuclei are ~10^{-4} yr^{-1}, then about 10% of the nuclei should exhibit the long-term UV/optical emission at a level of ~10^{38} ergs~s^{-1}.