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Abstract 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 ∼10³days. 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 ∼10³⁷–10³⁹erg 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 ∼10²days. 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 ∼10⁴⁷–10⁴⁹erg. 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. 

Abstract We present results from an extensive follow-up campaign of the tidal disruption event (TDE) ASASSN-15oi spanningδt ∼ 10–3000 days, offering an unprecedented window into the multiwavelength properties of a TDE during its first ≈8 yr of evolution. ASASSN-15oi is one of the few TDEs with strong detections at X-ray, optical/UV, and radio wavelengths and it also featured two delayed radio flares atδt ∼ 180 days andδt ∼ 1400 days. Our observations atδt > 1400 days reveal an absence of thermal X-rays, a late-time variability in the nonthermal X-ray emission, and sharp declines in the nonthermal X-ray and radio emission atδt ∼ 2800 days and ∼3000 days, respectively. The UV emission shows no significant evolution atδt > 400 days and remains above the pre-TDE level. We show that a cooling envelope model can explain the thermal emission consistently across all epochs. We also find that a scenario involving episodic ejection of material due to stream–stream collisions can possibly explain the first radio flare. Given the peculiar spectral and temporal evolution of the late-time emission, however, constraining the origins of the second radio flare and the nonthermal X-rays remains challenging. Our study underscores the critical role of long-term, multiwavelength follow-up to fully characterize the extended evolutionary phases of a TDE. 

Abstract We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of tidal disruption event (TDE) AT2018hyz, spanning 970–1300 d after optical discovery. In conjunction with earlier deeper limits, including those at ≈700 days, our observations reveal rapidly rising emission at 0.8–240 GHz, steeper than F ν ∝ t 5 relative to the time of optical discovery. Such a steep rise cannot be explained in any reasonable scenario of an outflow launched at the time of disruption (e.g., off-axis jet, sudden increase in the ambient density), and instead points to a delayed launch. Our multifrequency data allow us to directly determine the radius and energy of the radio-emitting outflow, and we find from our modeling that the outflow was launched ≈750 days after optical discovery. The outflow velocity is mildly relativistic, with β ≈ 0.25 and ≈0.6 for a spherical geometry and a 10° jet geometry, respectively, and the minimum kinetic energy is E K ≈ 5.8 × 10 49 and ≈6.3 × 10 49 erg, respectively. This is the first definitive evidence for the production of a delayed mildly relativistic outflow in a TDE; a comparison to the recently published radio light curve of ASASSN-15oi suggests that the final rebrightening observed in that event (at a single frequency and time) may be due to a similar outflow with a comparable velocity and energy. Finally, we note that the energy and velocity of the delayed outflow in AT2018hyz are intermediate between those of past nonrelativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE Sw J1644+57. We suggest that such delayed outflows may be common in TDEs. 

ABSTRACT Neutron star mergers produce a substantial amount of fast-moving ejecta, expanding outwardly for years after the merger. The interaction of these ejecta with the surrounding medium may produce a weak isotropic radio remnant, detectable in relatively nearby events. We use late-time radio observations of short duration gamma-ray bursts (sGRBs) to constrain this model. Two samples of events were studied: four sGRBs that are possibly in the local (<200 Mpc) Universe were selected to constrain the remnant non-thermal emission from the sub-relativistic ejecta, whereas 17 sGRBs at cosmological distances were used to constrain the presence of a proto-magnetar central engine, possibly re-energizing the merger ejecta. We consider the case of GRB 170817A/GW170817 and find that in this case the early radio emission may be quenched by the jet blast-wave. In all cases, for ejecta mass range of $${M}_{\rm {ej}}\lesssim 10^{-2}\, (5\times 10^{-2})\, \mathrm{M}_\odot$$, we can rule out very energetic merger ejecta $${E}_{\rm {ej}}\gtrsim 5\times 10^{52}\, (10^{53})\, \rm erg$$, thus excluding the presence of a powerful magnetar as a merger remnant.