Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. IV. Continued Fading and Non-Relativistic Expansion


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We present continued radio and X-ray observations of the previously relativistic tidal disruption event (TDE) Swift J164449.3+573451 (sw) extending to about 9.4 years post disruption, as part of ongoing campaigns with the Jansky Very Large Array (VLA) and the textit{Chandra} X-ray observatory. We find that the X-ray emission has faded below detectable levels, with an upper limit of $lesssim 3.5times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in a 100 ks observation, while the radio emission continues to be detected and steadily fade. Both are consistent with forward shock emission from a non-relativistic outflow, although we find that the radio spectral energy distribution is better fit at these late times with an electron power law index of $papprox 3$ (as opposed to $papprox 2.5$ at earlier times). With the revised spectral index we find $epsilon_Bapprox 0.01$ using the radio and X-ray data, and a density of $approx 0.04$ cm$^{3}$ at a radius of $Rapprox 0.65$ pc ($R_{rm sch}approx 2times 10^6$ R$_odot$) from the black hole. The energy scale of the blastwave is $approx 10^{52}$ erg. We also report detections of sw at 3 GHz from the first two epochs of the VLA Sky Survey (VLASS), and find that $sim 10^2$ off-axis sw-like events to $zsim 0.5$ may be present in the VLASS data. Finally, we find that sw itself will remain detectable for decades at radio frequencies, although observations at sub-GHz frequencies will become increasingly important to characterize its dynamical evolution.

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