2024ApJ...971..185C

We present radio observations of 23 optically discovered tidal disruption events (TDEs) on timescales of ∼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 ∼10³ days. While the radio emission in some of the detected TDEs peaked on a timescale ≈2-4 yr, over half of the sample still show rising emission. The range of luminosities for the sample is ∼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 ∼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 ∼10² 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 ∼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.