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

T. Eftekhari; E. Berger; B. A. Zauderer; R. Margutti; K. D. Alexander

doi:10.3847/1538-4357/aaa8e0

paper

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

arXiv:1710.07289 · doi:10.3847/1538-4357/aaa8e0

Abstract

We present continued radio and X-ray observations of the relativistic tidal disruption event Swift J164449.3+573451 extending to $δ t \approx 2000$ 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 $Chandra$ X-ray observations and demonstrate that the X-ray emission following the sharp decline at $δ t \approx 500$ 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 $δ t \approx 10 - 200$ d, corresponding to a magnetic field energy density fraction of $ε_{B} \approx 1 0^{- 3}$ , well below equipartition; the X-ray data demonstrate that this deviation from equipartition holds to at least $δ t \approx 2000$ 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 $E_{K} \approx 4 \times 1 0^{51}$ erg and a transition to non-relativistic expansion on the timescale of our latest observations ( $δ t \approx 700$ d). 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 $z \sim 2$ , but with a slow evolution that may inhibit their recognition as transient events.

Submitted to ApJ

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

Abstract

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