Late-time Evolution and Instabilities of Tidal Disruption Disks

Observations of tidal disruption events (TDEs) on a timescale of years after the main flare show evidence of continued activity in the form of optical/UV emission, quasi-periodic eruptions, and delayed radio flares. Motivated by this, we explore the time evolution of these disks using semi-analytic models to follow the changing disk properties and feeding rate to the central black hole (BH). We find that thermal instabilities typically begin $\sim100\,{\rm days}$ after the TDE, causing the disk to cycle between high and low accretion states for up to $\sim10\,{\rm yrs}$. The high state is super-Eddington, which may be associated with outflows that eject $\sim10^{-3}-10^{-1}\,M_\odot$ over $\sim1-2\,{\rm days}$ with a range of velocities of $\sim0.03-0.3c$. Collision between these mass ejections may cause radio flares. In the low state, the accretion rate slowly grows over months to years as continued fallback accretion builds the disk's mass. In this phase, the disk has a luminosity of $\sim10^{41}-10^{42}\,{\rm erg\,s^{-1}}$ in the optical/UV as seen in some late-time observations. Although the accretion cycles we find occur for a typical $\alpha$-disk, in nature the disk could be stabilized by other effects such as the disk's magnetic field or heating from fallback accretion, the latter of which we explore. Thus higher cadence optical/UV observations along with joint radio monitoring will be key for following the disk state and testing these models.