PREPRINT
F6A2E165-95A4-4002-9473-34D347706527

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

Yvette Cendes, Edo Berger, Kate Alexander, Sebastian Gomez, Aprajita Hajela, Ryan Chornock, Tanmoy Laskar, Raffaella Margutti, Brian Metzger, Michael Bietenholz, Daniel Brethauer, Mark Wieringa
arXiv:2206.14297

Submitted on 28 June 2022

## 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 ${E}_{K}\approx 5.8×{10}^{49}$ and $\approx 6.3×{10}^{49}$ 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 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.

## Preprint

Comment: Submitted to ApJ

Subject: Astrophysics - High Energy Astrophysical Phenomena