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

We present continued radio and X-ray observations of the previously relativistic tidal disruption event (TDE) Swift J164449.3+573451 (Sw 1644+57) extending to about 9.4 yr post disruption, as part of ongoing campaigns with the Jansky Very Large Array (VLA) and the Chandra X-ray observatory. We find that the X-ray emission has faded below detectable levels, with an upper limit of 3.5 × 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 p 3 (as opposed to p 2.5 at earlier times). With the revised spectral index we find B 0.01 using the radio and X-ray data, and a density of 0.04 cm3 at a radius of R 0.65 pc (Rsch 2 × 106 R ) from the black hole. The energy scale of the blastwave is 1052 erg. We also report detections of Sw 1644+57 at 3 GHz from the first two epochs of the VLA Sky Survey (VLASS), and find that ∼102 off-axis Sw 1644+57-like events to z ∼ 0.5 may be present in the VLASS data. Finally, we find that Sw 1644+57 itself will remain detectable for decades at radio frequencies, although observations at sub-GHz frequencies will become increasingly important to characterize its dynamical evolution.