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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 SwiftJ164449.3+573451 extending to \sim216 days after discovery. The data are 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 \sim1 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\sim0.4-0.6 pc. The increase in energy cannot 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) \propto Gamma^{-2.5}. The similar ratio of duration to dynamical timescale for Sw1644+57 and GRBs suggests that this result may be applicable to GRBs as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at r\sim0.5 pc in good agreement with the Bondi radius for a \sim10^6 M_sun black hole. The density at \sim0.5 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 (\sim216 d) we find that the jet energy is E_{iso}\sim5x10^{53} erg (E_j\sim2.4x10^{51} erg for theta_j=0.1), the radius is r\sim1.2 pc, the Lorentz factor is Gamma\sim2.2, the ambient density is n\sim0.2 cm^{-3}, and the projected size is r_{proj}\sim25 microarcsec. Assuming no future changes in the observed evolution we predict that the radio emission from Sw1644+57 should be detectable with the EVLA for several decades, and will be resolvable with VLBI in a few years.