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

TL;DR

This paper reports the discovery of a delayed, mildly relativistic outflow in the TDE AT2018hyz, launched approximately 750 days after the initial disruption, with implications for understanding outflow mechanisms in tidal disruption events.

Contribution

It provides the first definitive evidence of a delayed mildly-relativistic outflow in a TDE, with detailed measurements of its velocity, energy, and launch timing, expanding knowledge of TDE outflow diversity.

Findings

Outflow launched approximately 750 days after optical discovery.

Outflow velocity is mildly relativistic, with 225 to 26 for different geometries.

Minimum kinetic energy of the outflow is around 6 7 10^{49} erg.

Abstract

We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of the tidal disruption event (TDE) AT2018hyz, spanning $970 - 1300$ d after optical discovery. In conjunction with earlier deeper limits, including at $\approx 700$ d, our observations reveal rapidly rising emission at $0.8-240$ GHz, steeper than $F_\nu\propto t^5$ 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 multi-frequency data allow us to directly determine the radius and energy of the radio-emitting outflow, showing that it was launched $\approx 750$ d after optical discovery. The outflow velocity is mildly relativistic, with $\beta\approx 0.25$ and $\approx 0.6$ for a spherical and a $10^\circ$ jet geometry, respectively, and the minimum kinetic energy is $E_K\approx 5.8\times 10^{49}$ and $\approx 6.3\times 10^{49}$ 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 re-brightening 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 non-relativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE Sw\,J1644+57. We suggest that such delayed outflows may be common in TDEs.