A Mildly Relativistic Outflow Launched Two Years after Disruption in Tidal Disruption Event AT2018hyz

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 approximate to 700 days, our observations reveal rapidly rising emission at 0.8-240 GHz, steeper than F ( nu ) proportional to 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 approximate to 750 days after optical discovery. The outflow velocity is mildly relativistic, with beta approximate to 0.25 and approximate to 0.6 for a spherical geometry and a 10 degrees jet geometry, respectively, and the minimum kinetic energy is E ( K ) approximate to 5.8 x 10(49) and approximate to 6.3 x 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.