2013ApJ...767..152Z

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 R_s 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 ∼ {dot}M_Edd ~ 0.006 M_☉/yr (for a 3x10⁶ M_☉ black hole and order unity efficiency) indicating that the peak accretion rate was about 330 {dot}M_Edd, 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 x10⁵⁴ erg (E_j~ 10⁵² erg for a jet opening angle, θ_ 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.