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 δt 216days 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 ρr ^-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 Lt ^-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 Sw1644+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 × 10⁶ 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 216days) we find that the jet energy is E _{j, iso} 5 × 10⁵³ erg (E_j 2.4 × 10⁵¹ 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 cm ^-3, 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 10 ⁵² erg, we predict that the radio emission from Sw1644+57 should be detectable with the EVLA for several decades and will be resolvable with very long baseline interferometry in a few years.