2012ApJ...748...36B

We present continued radio observations of the tidal disruption event Swift J164449.3+573451 extending to δt ~ 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 δt ~ 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 ρ∝r ^–3/2 (0.1-1.2 pc) with a significant flattening at r ~ 0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L∝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(> Γ_j)∝Γ^–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 ∼fewx10⁶ 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 (δt ~ 216 days) we find that the jet energy is E_j, iso_~ 5x10⁵³ erg (E_j~ 2.4x10⁵¹ erg for θ_j= 0.1), the radius is r ~ 1.2 pc, the Lorentz factor is Γ_j~ 2.2, the ambient density is n ~ 0.2/cm3, and the projected angular size is r_proj~ 25 µas, below the resolution of the VLBA+Effelsberg. Assuming no future changes in the observed evolution and a final integrated total energy of E_j~ 10⁵² 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.