Radio Monitoring of the Tidal Disruption Event Swift j164449.3+573451. Ii. the Relativistic Jet Shuts Off and a Transition to Forward Shock X-Ray/radio Emission

Abstract

We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to δt ≈ 600 d. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsecscale environment around the disrupting supermassive black hole (SMBH). We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at ≈ 500 d, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of δt/t . 0.2 (and by a factor of 15 in δt/t ≈ 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at . 500 d, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at ≈ 610 d is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of ∼ 30 − 60 eV and inner radius of ∼ 2 − 10Rs cannot accommodate the observed flux level or the detected emission at & 1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below ∼ M˙ Edd ≈ 0.006 M⊙ yr−1 (for a 3 × 106 M⊙ black hole and order unity efficiency) indicating that the peak accretion rate was about 330M˙ Edd, and the total accreted mass by δt ≈ 500 d is about 0.15 M⊙. From the radio data we further find significant flattening in the integrated energy of the forward shock at δt & 250 d with Ej,iso ≈ 2 × 1054 erg (Ej ≈ 1052 erg for a jet opening angle, θj = 0.1) following a rise by about a factor of 15 at ≈ 30 − 250 d. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.