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We model the non-thermal transient Swift J1644+57 as resulting from arelativistic jet powered by the accretion of a tidally-disrupted star onto asuper-massive black hole. Accompanying synchrotron radio emission is producedby the shock interaction between the jet and the dense circumnuclear medium,similar to a gamma-ray burst afterglow. An open mystery, however, is the originof the late-time radio rebrightening, which occurred well after the peak of thejetted X-ray emission. Here, we systematically explore several proposedexplanations for this behavior by means of multi-dimensional hydrodynamicsimulations coupled to a self-consistent radiative transfer calculation of thesynchrotron emission. Our main conclusion is that the radio afterglow of SwiftJ1644+57 is not naturally explained by a jet with a one-dimensional top-hatangular structure. However, a more complex angular structure comprised of anultra-relativistic core (Lorentz factor $\Gamma \sim 10$) surrounded by aslower ($\Gamma \sim $ 2) sheath provides a reasonable fit to the data. Such ageometry could result from the radial structure of the super-Eddingtonaccretion flow or as the result of jet precession. The total kinetic energy ofthe ejecta that we infer of $\sim$ few $10^{53}\,$erg requires a highlyefficient jet launching mechanism. Our jet model providing the best fit to thelight curve of the on-axis event Swift J1644+57 is used to predict the radiolight curves for off-axis viewing angles. Implications for the presence ofrelativistic jets from TDEs detected via their thermal disk emission, as wellas the prospects for detecting orphan TDE afterglows with upcoming wide-fieldradio surveys and resolving the jet structure with long baselineinterferometry, are discussed.

The radio afterglow of Swift J1644+57 reveals a powerful jet with fast core and slow sheath