The multiwavelength context of delayed radio emission in tidal disruption events: evidence for accretion-driven outflows

Recent observations presented in Y. Cendes et al. show that optically selected tidal disruption events (TDEs) commonly produce delayed radio emission that can peak years after disruption. Here, we explore the multiwavelength properties of a sample of radio-observed optically selected TDEs, to shed light on the physical process(es) responsible for the late-rising radio emission. We combine new late-time X-ray observations with archival optical, UV, X-ray, and radio data to conclude that a diversity of accretion-driven outflows may power the delayed radio emission in TDEs. Our analysis suggests that some late radio outflows may be launched by a delayed phase of super-Eddington accretion onto the central supermassive black hole (SMBH), while others may result from a state transition to a “low–hard” radiatively inefficient accretion flow or the deceleration of an off-axis relativistic jet. We find that TDEs with delayed radio emission are less likely to exhibit helium emission lines at early times (p = 0.002) and may have larger optical/UV photospheric radii (p = 0.026) than other TDEs, possibly also indicating that the onset of SMBH accretion is delayed in these systems. Our results have implications for our understanding of state changes in SMBH accretion flows, the circularization timescale for TDE debris, and the prevalence of off-axis jets in TDEs, and they motivate systematic long-term monitoring of these unique transients. The objects in our sample with the brightest radio emission are also detected in the Very Large Array Sky Survey, demonstrating that all-sky radio surveys can play an important role in discovering unexpected properties of the TDE population.