The Multi-Wavelength Context of Delayed Radio Emission in TDEs: Evidence for Accretion-Driven Outflows

TL;DR

The paper tackles the origin of delayed radio emission in tidal disruption events (TDEs) by performing a multi-wavelength analysis of a large, optically selected sample with new late-time X-ray data. It tests two limiting accretion scenarios: Case 1 where rapid circularization makes the optical light trace the instantaneous accretion rate $\dot{M}$, and Case 2 where accretion is substantially delayed, affecting radio launch timing. The findings support a diversity of accretion-driven outflows powering delayed radio emission, including potential launches during delayed super-Eddington accretion, state transitions to radiatively inefficient flows, or decelerating off-axis jets, with additional links to helium line strength and optical/UV photospheric radii; notably, the brightest radio TDEs are detectable in all-sky surveys like VLASS, highlighting the value of wide-field radio monitoring. The work advances our understanding of SMBH accretion-state changes and debris circularization timescales in TDEs and provides a public, multi-wavelength dataset for future population studies via OTTER.

Abstract

Recent observations presented in Cendes et al. (2024a) show that optically selected tidal disruption events (TDEs) commonly produce delayed radio emission that can peak years post-disruption. Here, we explore the multi-wavelength 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 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 motivate systematic, long-term monitoring of these unique transients. The objects in our sample with the brightest radio emission are also detected in the VLA Sky Survey (VLASS), demonstrating that all-sky radio surveys can play an important role in discovering unexpected properties of the TDE population.