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 δ t ≈ 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 δ t ≈ 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 ∼few × 10 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 (δ t ≈ 216 days) we find that the jet energy is E j , iso ≈ 5 × 10 53 erg ( E j ≈ 2.4 × 10 51 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 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.