Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. III. Late-time Jet Energetics and a Deviation from Equipartition

Abstract

We present continued radio and X-ray observations of the relativistic tidal disruption event Swift J164449.3+573451 extending to $\delta t \approx 2000$ d after discovery. The radio data were obtained with the VLA as part of a long-term program to monitor the energy and dynamical evolution of the relativistic jet and to characterize the parsec-scale environment around a previously dormant supermassive black hole. We combine these data with $\textit{Chandra}$ X-ray observations and demonstrate that the X-ray emission following the sharp decline at $\delta t \approx 500$ d is due to the forward shock. Using the X-ray data, in conjunction with optical/NIR data, we constrain the synchrotron cooling frequency and the microphysical properties of the outflow for the first time. We find that the cooling frequency evolves through the optical/NIR band at $\delta t \approx 10 - 200$ d, corresponding to a magnetic field energy density fraction of $\epsilon_B \approx 10^{-3}$, well below equipartition; the X-ray data demonstrate that this deviation from equipartition holds to at least $\delta t \approx 2000$ d. We thus recalculate the physical properties of the jet over the lifetime of the event, no longer assuming equipartition. We find a total kinetic energy of $E_K \approx 4 \times 10^{51}$ erg and a transition to non-relativistic expansion on the timescale of our latest observations ($\delta t \approx 700$ d). The density profile is approximately $R^{-3/2}$ at $\lesssim 0.3$ pc and $\gtrsim 0.7$ pc, with a plateau at intermediate scales, characteristic of Bondi accretion. Based on its evolution thus far, we predict that Sw 1644+57 will be detectable at centimeter wavelengths for decades to centuries with existing and upcoming radio facilities. Similar off-axis events should be detectable to $z \sim 2$, but with a slow evolution that may inhibit their recognition as transient events.