Ubiquitous Late Radio Emission from Tidal Disruption Events

We present radio observations of 23 optically-discovered tidal disruption events (TDEs) on timescales of about 500-3200 days post-discovery. We detect 9 new TDEs that did not have detectable radio emission at earlier times, indicating a late-time brightening after several hundred (and up to 2300 days); an additional 6 TDEs exhibit radio emission whose origin is ambiguous or may be attributed to the host galaxy or an AGN. We also report new rising components in two TDEs previously detected in the radio (iPTF16fnl and AT2019dsg) at ~1000 days. While the radio emission in some of the detected TDEs peaked on a timescale of ~2-4 years, more than half of the sample still shows rising emission. The range of luminosities for the sample is 10^37-10^39 erg/s, about two orders of magnitude below the radio luminosity of the relativistic TDE Sw1644+57. Our data set indicates that about 40% of all optical TDEs are detected in the radio hundreds to thousands of days after discovery, and that this is probably more common than early radio emission peaking at ~100 days. Using an equipartition analysis, we find evidence for a delayed launch of the radio-emitting outflows, with delay timescales of ~500-2000 days, inferred velocities of ~0.02-0.15c, and kinetic energies of ~10^47-10^49 erg. We rule out off-axis relativistic jets as a viable explanation for this population, and conclude delayed outflows are a more likely explanation, such as from delayed disk formation. Finally, we find comparable densities in the circumnuclear environments of these TDEs as for those with early radio emission, and find the TDEs still rising in luminosity are consistent with free expansion. We conclude that late radio emission marks a fairly ubiquitous but heretofore overlooked phase of TDE evolution.