We present continued radio observations of the tidal disruption event Swift J164449.3+573451 extending to \delta t \approx 216 days after discovery .
The data were obtained with the EVLA , AMI Large Array , CARMA , the SMA , and the VLBA+Effelsberg as part of a long-term program to monitor the expansion and energy scale of the relativistic outflow , and to trace the parsec-scale environment around a previously-dormant supermassive black hole ( SMBH ) .
The new observations reveal a significant change in the radio evolution starting at \delta t \approx 1 month , with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude , and an overall density profile around the SMBH of \rho \propto r ^ { -3 / 2 } ( 0.1 - 1.2 pc ) with a significant flattening at r \approx 0.4 - 0.6 pc .
The increase in energy can not be explained with continuous injection from an L \propto t ^ { -5 / 3 } tail , which is observed in the X-rays .
Instead , we conclude that the relativistic jet was launched with a wide range of Lorentz factors , obeying E ( > \Gamma _ { j } ) \propto \Gamma _ { j } ^ { -2.5 } .
The similar ratio of duration to dynamical timescale for Sw 1644+57 and GRBs suggests that this result may be applicable to GRB jets as well .
The radial density profile may be indicative of Bondi accretion , with the inferred flattening at r \sim 0.5 pc in good agreement with the Bondi radius for a \sim { few } \times 10 ^ { 6 } M _ { \odot } black hole .
The density at \sim 0.5 pc is about a factor of 30 times lower than inferred for the Milky Way galactic center , potentially due to a smaller number of mass-shedding massive stars .
From our latest observations ( \delta t \approx 216 d ) we find that the jet energy is E _ { j,iso } \approx 5 \times 10 ^ { 53 } erg ( E _ { j } \approx 2.4 \times 10 ^ { 51 } erg for \theta _ { j } = 0.1 ) , the radius is r \approx 1.2 pc , the Lorentz factor is \Gamma _ { j } \approx 2.2 , the ambient density is n \approx 0.2 cm ^ { -3 } , and the projected angular size is r _ { proj } \approx 25 \mu as , below the resolution of the VLBA+Effelsberg .
Assuming no future changes in the observed evolution and a final integrated total energy of E _ { j } \approx 10 ^ { 52 } erg , we predict that the radio emission from Sw 1644+57 should be detectable with the EVLA for several decades , and will be resolvable with VLBI in a few years .