[Paper Review] What powers the radio emission in TDE AT2019dsg: a long-lived jet or the disruption itself?

Tatsuya Matsumoto, Tsvi Piran+1 more|arXiv (Cornell University)|Sep 6, 2021

Astrophysics and Cosmic PhenomenaPhysics and Astronomy47 references23 citations

TL;DR

This paper argues that the long-lived radio emission in tidal disruption event AT2019dsg is powered by shock heating from a single, impulsive ejection of stellar debris, not continuous energy injection from a central black hole engine. The observed increase in radio energy over time arises naturally from kinetic energy transfer as the outflow sweeps up circumnuclear material, with the equipartition model showing consistent velocity and energy growth that matches a freely-coasting or weakly decelerating shock. The key result is that continuous energy injection is unnecessary and requires fine-tuning, while the shock-driven model provides a simpler, self-consistent explanation for the radio light curve and spectral evolution.

ABSTRACT

The tidal disruption event AT2019dsg was observed from radio to X-rays and was possibly accompanied by a high-energy neutrino. Previous interpretations have focused on continued injection by a central engine as the source of energy for radio emission. We show that continuous energy injection is unnecessary; the radio data can be explained by a single ejection of plasma that supplies all the energy needed. To support this assertion, we analyze the synchrotron self-absorbed spectra in terms of the equipartition model. Similar to previous analyses, we find that the energy in the radio-emitting region increases approximately $\propto t^{0.7}$ and the lengthscale of this region grows $\propto t$ at a rate $\simeq0.06c$. This event resembles the earliest stage of a supernova remnant: because the ejected mass is much greater than the shocked external mass, its velocity remains unchanged, while the energy in shocked gas grows with time. The radio-emitting material gains energy from the outflow, not continuing energy injection by the central object. Although energy injection from an accreting BH cannot be completely excluded, the energy injection rate is very different from the fallback luminosity, and maintaining constant outflow velocity requires fine-tuning demanding further physical explanation. If the neutrino association is real, the energy injection needed is much greater than for the radio emission, suggesting that the detected neutrino did not arise from the radio-emitting region.

Motivation & Objective

• To determine the physical origin of the long-lived radio emission in TDE AT2019dsg.
• To test whether continuous energy injection from the central black hole is required to explain the observed increase in radio energy over time.
• To assess the viability of a shock-driven outflow model versus a persistent jet model for powering the radio emission.
• To evaluate the implications for the reported high-energy neutrino detection in relation to the radio-emitting region.

Proposed method

• Applied the equipartition model to synchrotron self-absorbed (SSA) radio spectra to estimate the radius, energy, and magnetic field strength in the radio-emitting region at each observation time.
• Used corrected expressions for equipartition radius and energy that include the deep-Newtonian dependence of electron energy on radius, improving accuracy over prior analyses.
• Fitted the time evolution of the SSA peak frequency and luminosity with both freely-coasting and decelerating outflow models to infer velocity and circumnuclear medium (CNM) density profiles.
• Compared the inferred energy injection rate from the central engine with the fallback luminosity and found a significant mismatch, indicating fine-tuning is required for continuous injection.
• Assessed the consistency of the shock model with VLBI observations, which show no evidence of relativistic outflows.
• Evaluated the neutrino connection by comparing required jet luminosities for neutrino production with those inferred from radio emission, finding a large discrepancy.

Experimental results

Research questions

• RQ1 Is continuous energy injection from the central black hole necessary to explain the increasing radio luminosity in AT2019dsg?
• RQ2 Can the observed time evolution of the radio spectral peak be explained by a single ejection of plasma without ongoing central activity?
• RQ3 What is the implied circumnuclear medium (CNM) density profile, and how does it constrain the outflow dynamics?
• RQ4 Is the high-energy neutrino detected by IceCube associated with the radio-emitting region, and does the required energy budget support this?
• RQ5 Does the shock-driven model provide a simpler and more physically consistent explanation than a persistent jet?

Key findings

• The equipartition radius increases with time as ∝t, indicating a shock-driven outflow with a velocity of approximately 13,000 km s−1 for a freely-coasting model.
• The energy in the radio-emitting region increases as ∝t^0.7, consistent with shock heating from swept-up circumnuclear material rather than continuous injection.
• The inferred circumnuclear medium (CNM) density is approximately 5 times lower than previously estimated by Cendes et al. (2021), due to inclusion of the deep-Newtonian correction in the equipartition formalism.
• Continuous energy injection would require a finely-tuned energy input rate that does not match the fallback luminosity and would produce an unobserved reverse shock.
• The required jet luminosity to produce the detected neutrino is ~3×10^44 erg s−1, which is three orders of magnitude higher than the energy budget inferred from radio emission, making a direct link implausible.
• The observed radio evolution is most naturally explained by a single ejection of debris whose kinetic energy is gradually transferred to the shocked region, analogous to the early phase of a supernova remnant.