[Paper Review] A Mildly Relativistic Outflow Launched Two Years after Disruption in the Tidal Disruption Event AT2018hyz

Y. Cendes, E. Berger+10 more|arXiv (Cornell University)|Jun 28, 2022

Astrophysics and Cosmic PhenomenaPhysics and Astronomy55 references70 citations

TL;DR

This study presents the first definitive detection of a delayed, mildly relativistic outflow in a tidal disruption event (TDE), AT2018hyz, launched approximately 750 days after optical discovery. Multi-frequency radio, mm, optical/UV, and X-ray observations reveal a rapidly rising radio light curve steeper than Fν ∝ t⁵, indicating a delayed outflow with β ≈ 0.25–0.6 and kinetic energy EK ≈ 5.8–6.3 × 10⁴⁹ erg, ruling out prompt jet or ambient density enhancement models.

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 $F_ u\propto t^5$ 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 $E_K\approx 5.8 imes 10^{49}$ and $\approx 6.3 imes 10^{49}$ 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.

Motivation & Objective

• To investigate the origin of late-time, rapidly rising radio emission in the tidal disruption event AT2018hyz, observed 970–1300 days after optical discovery.
• To distinguish between competing models for delayed radio emission, including off-axis relativistic jets, ambient density enhancements, and delayed outflow launch.
• To determine the physical properties of the radio-emitting outflow, including its radius, velocity, energy, and geometry, using multi-frequency radio and SED data.

Proposed method

• Multi-frequency radio and mm observations were conducted using the VLA, ALMA, AMI-LA, and ASKAP, spanning 0.8–240 GHz, with deep upper limits at ~700 days.
• Spectral energy distribution (SED) modeling was performed using a synchrotron self-absorption model with equipartition assumptions to derive outflow parameters such as radius, Lorentz factor, and kinetic energy.
• A two-outflow model was tested to account for the unusual spectral evolution, with one component peaking at ~1.5 GHz and another at ~3 GHz, to assess temporal evolution and emission dominance.
• The data were analyzed using MCMC sampling with priors enforcing physical consistency in the time evolution of peak frequency and flux density.
• Outflow geometry was modeled for both spherical and 10°-jet configurations to assess the impact on derived velocity and energy estimates.
• X-ray and optical/UV data were used to cross-check the timing and energetics of the outflow relative to accretion and fallback processes.

Experimental results

Research questions

• RQ1 What mechanism explains the rapid rise in radio luminosity observed in AT2018hyz at 970–1300 days post-optical discovery?
• RQ2 Can the observed radio light curve be explained by a prompt relativistic jet, ambient medium interaction, or delayed outflow launch?
• RQ3 What are the physical parameters (velocity, energy, radius) of the radio-emitting outflow, and how do they depend on outflow geometry?
• RQ4 Is the circumnuclear environment of AT2018hyz unusually dense, or is the delay due to internal accretion disk processes?
• RQ5 How does the delayed outflow in AT2018hyz compare to other TDEs like ASASSN-15oi and Sw J1644+57 in terms of energy and velocity?

Key findings

• The radio emission in AT2018hyz exhibits a steep rise steeper than Fν ∝ t⁵, inconsistent with prompt outflow models such as off-axis jets or sudden ambient density enhancements.
• The outflow was launched approximately 750 days after optical discovery, with a mildly relativistic velocity of β ≈ 0.25 for a spherical outflow and β ≈ 0.6 for a 10°-jet geometry.
• The minimum kinetic energy of the outflow is EK ≈ 5.8 × 10⁴⁹ erg (spherical) and ≈6.3 × 10⁴⁹ erg (jet), placing it between non-relativistic TDEs and the relativistic TDE Sw J1644+57.
• The circumnuclear environment has typical density for TDEs, ruling out interaction with a dense medium as the cause of the delay.
• A two-outflow model was tested but did not resolve the rapid evolution; both components still require a delayed, mildly relativistic origin.
• This is the first definitive evidence of a delayed mildly relativistic outflow in a TDE, suggesting such outflows may be common in the TDE population.