Ubiquitous Late Radio Emission from Tidal Disruption Events

We present radio observations of 23 optically discovered tidal disruption events (TDEs) on timescales of similar to 500-3200 days postdiscovery. We detect nine 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 seven TDEs exhibit radio emission whose origin is ambiguous or may be attributed to the host galaxy or an active galactic nucleus. We also report a new rising component in one TDE previously detected in the radio at similar to 103 days. While the radio emission in some of the detected TDEs peaked on a timescale approximate to 2-4 yr, over half of the sample still show rising emission. The range of luminosities for the sample is similar to 1037-1039 erg s-1, about 2 orders of magnitude below the radio luminosity of the relativistic TDE Sw J1644+57. Our data set indicates similar to 40% of all optical TDEs are detected in radio hundreds to thousands of days after discovery, and that this is probably more common than early radio emission peaking at similar to 102 days. Using an equipartition analysis, we find evidence for a delayed launch of the radio-emitting outflows, with delay timescales of similar to 500-2000 days, inferred velocities of approximate to 0.02-0.15c, and kinetic energies of similar to 1047-1049 erg. We rule out off-axis relativistic jets as a viable explanation for this population, and conclude delayed outflows are a more likely explanation, possibly from delayed disk formation. We conclude late radio emission marks a fairly ubiquitous but heretofore overlooked phase of TDE evolution.