We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 ( Sw 1644+57 ) extending to \delta t \approx 600 d. 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 ( SMBH ) .
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 \approx 500 d , with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of \delta t / t \lesssim 0.2 ( and by a factor of 15 in \delta t / t \approx 0.05 ) .
The rapid decline rules out a forward shock origin ( direct or reprocessing ) for the X-ray emission at \lesssim 500 d , 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 \approx 610 d 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 \sim 30 - 60 eV and inner radius of \sim 2 - 10 R _ { s } can not accommodate the observed flux level or the detected emission at \gtrsim 1 keV .
We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below \sim \dot { M } _ { Edd } \approx 0.006 M _ { \odot } yr ^ { -1 } ( for a 3 \times 10 ^ { 6 } M _ { \odot } black hole and order unity efficiency ) indicating that the peak accretion rate was about 330 \dot { M } _ { Edd } , and the total accreted mass by \delta t \approx 500 d is about 0.15 M _ { \odot } .
From the radio data we further find significant flattening in the integrated energy of the forward shock at \delta t \gtrsim 250 d with E _ { j,iso } \approx 2 \times 10 ^ { 54 } erg ( E _ { j } \approx 10 ^ { 52 } erg for a jet opening angle , \theta _ { j } = 0.1 ) following a rise by about a factor of 15 at \approx 30 - 250 d. Projecting forward , we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates .