Radio monitoring of the tidal disruption event swift J164449.3+573451. II. the relativistic jet shuts off and a transition to forward shock x-ray/radio emission

We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to t 600 days. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsec-scale environment around the disrupting supermassive black hole. We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at 500 days, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of δt/t ≲ 0.2 (and by a factor of 15 in δt/t 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at ≲ 500 days, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at 610 days is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of 30-60 eV and inner radius of 2-10 Rs cannot accommodate the observed flux level or the detected emission at ≳ 1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below M_⊙ yr^-1 (for a 3 × 10⁶ M_⊙ black hole and order unity efficiency) indicating that the peak accretion rate was about , and the total accreted mass by t 500 days is about 0.15 M . From the radio data we further find significant flattening in the integrated energy of the forward shock at t ≳ 250 days with E_j,iso 2 × 10⁵⁴ erg (E_j 10 ⁵² erg for a jet opening angle, j = 0.1) following a rise by about a factor of 15 at 30-250 days. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.