RADIO MONITORING OF THE TIDAL DISRUPTION EVENT SWIFT J164449.3+573451. I. JET ENERGETICS AND THE PRISTINE PARSEC-SCALE ENVIRONMENT OF A SUPERMASSIVE BLACK HOLE

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] [is proportional to]r super(-3/2) (0.1-1.2 pc) with a significant flattening at r [approx =] 0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L [is proportional to]t super(-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)[is proportional to][Gamma] super(-2.5)j. The similar ratios of duration to dynamical timescale for Sw 1644+57 and gamma-ray bursts (GRBs) suggest 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 ~ 0.5 pc in good agreement with the Bondi radius for a ~few x 10 super(6) M black hole. The density at ~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 days) we find that the jet energy is E sub(j, iso) [approx =] 5 x 10 super(53) erg (E sub(j) [approx =] 2.4 x 10 super(51) erg or [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 super(-3), and the projected angular size is r sub(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 sub(j) [approx =] 10 super(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 very long baseline interferometry in a few years.