Delayed radio emission in tidal disruption events from collisions of outflows driven by disk instabilities

Abstract

Delayed radio emission has been associated with a growing proportion of tidal disruption events (TDEs). For many events, the radio synchrotron emission is inferred to originate from the interaction of mildly-relativistic outflows, launched with delay times of $\sim 100$--$1000$ d after the TDE optical peak. The mechanism behind these outflows remains uncertain, but may relate to instabilities or state transitions in the accretion disk formed from the TDE. We model the radio emission powered by the collision of mass outflows ("flares") from TDE accretion disks, considering scenarios in which two successive disk flares collide with each other, as well as collisions between the outflow and the circumnuclear medium (CNM). For flare masses of $\sim 0.01$-$0.1 M_{\odot}$, varied CNM densities, and different time intervals between ejected flares, we demonstrate that the shocks formed by the collisions have velocities $0.05c$-$0.3c$ at $\sim 10^{17}$ cm and power bright radio emission of $L_{\nu} \sim 10^{27}$-$10^{30}$ erg s$^{-1}$ Hz$^{-1}$, consistent with the properties inferred for observed events. We quantify how the typical peak timescale and flux varies for different properties of our models, and compare our model predictions to a selection of TDEs with delayed radio emission. Our models successfully reproduce the light curves and SEDs for several events, supporting the idea that delayed outflows from disk instabilities and state transitions can power late-time radio emission in TDEs.