One of the intriguing puzzles concerning Swift J1644+57, the first jetted tidal disruption event (TDE) discovered, is the constant increase in its jet energy, as implied by radio observations. During the first two hundred days, the jet energy has increased by an order of magnitude. We suggest that the jet was viewed slightly off-axis. In this case, the apparent energy increase arises due to the slowing down of the jet and the corresponding broadening of its beaming cone. Using equipartition analysis, we infer an increasing jet energy as a larger region of the jet is observed. A simple off-axis model accounts nicely for the multi-wavelength radio observations, resolving this long-standing puzzle. The model allows us to self-consistently evolve the synchrotron signature from an off-axis jet as a function of time. It also allows us to estimate, for the first time, the beaming angle of the jet, θ0 ≈ 21°. Considering existing limits on the black hole mass, ≲107 M, this angle implies that the prompt phase beaming corrected luminosity of Swift J1644+57, ∼1047 ergs sec-1, was super Eddington. We also present a closure relation between the spectral and temporal flux for off-axis jets, which can be used to test whether a given radio transient is off-axis or not.
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© 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.