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

Abstract

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(> Γj) ∝ Γ−2.5j. The similar ratio of duration to dynamical timescale for Sw 1644+57 and GRBs suggests 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 ≈ 216 d) we find that the jet energy is Ej,iso ≈ 5×1053 erg (Ej ≈ 2.4×1051 erg for θj = 0.1), the radius is r ≈ 1.2 pc, the Lorentz factor is Γj ≈ 2.2, the ambient density is n ≈ 0.2 cm−3, and the projected angular size is rproj ≈ 25 µas, below the resolution of the VLBA+Effelsberg. Assuming no future changes in the observed evolution and a final integrated total energy of Ej ≈ 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 VLBI in a few years.