Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. IV. Continued Fading and Non-Relativistic Expansion
Abstract
We present continued radio and X-ray observations of the previously relativistic tidal disruption event (TDE) Swift J164449.3+573451 () extending to about 9.4 years 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 εB≈ 0.01 using the radio and X-ray data, and a density of ≈ 0.04 cm3 at a radius of R≈ 0.65 pc (R sch≈ 2× 106 R) from the black hole. The energy scale of the blastwave is ≈ 1052 erg. We also report detections of \ at 3 GHz from the first two epochs of the VLA Sky Survey (VLASS), and find that 102 off-axis -like events to z 0.5 may be present in the VLASS data. Finally, we find that \ itself will remain detectable for decades at radio frequencies, although observations at sub-GHz frequencies will become increasingly important to characterize its dynamical evolution.