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 δt  ≈ 2000 days after discovery. The radio data were obtained with the Very Large Array (VLA) as part of a long-term program to monitor the energy and dynamical evolution of the jet and to characterize the parsec-scale environment around a previously dormant supermassive black hole. We combine these data with Chandra observations and demonstrate that the X-ray emission following the sharp decline at δt  ≈ 500 days is likely due to the forward shock. 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 δt  ≈ 10–200 days, corresponding to ϵ B  ≈ 10 −3 , well below equipartition; the X-ray data demonstrate that this deviation from equipartition holds to at least δt  ≈ 2000 days. 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  ≈ 4 × 10 51 erg and a transition to non-relativistic expansion on the timescale of our latest observations (700 days). The density profile is approximately R −3/2 at ≲0.3 pc and ≳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  ∼ 2, but with a slow evolution that may inhibit their recognition as transient events.