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 t ≈ 600 days. 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 parsec-scale environment around the disrupting supermassive black hole. 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 days, 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 days, 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 days 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 ~2-10 Rs 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 ∼\dot{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 330 \dot{M}_Edd, and the total accreted mass by t ≈ 500 days is about 0.15 M ⊙. From the radio data we further find significant flattening in the integrated energy of the forward shock at t >~ 250 days with E j, iso ≈ 2 × 1054 erg (Ej ≈ 1052 erg for a jet opening angle, θ j = 0.1) following a rise by about a factor of 15 at ≈30-250 days. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.
- Publication:
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The Astrophysical Journal
- Pub Date:
- April 2013
- DOI:
- 10.1088/0004-637X/767/2/152
- arXiv:
- arXiv:1212.1173
- Bibcode:
- 2013ApJ...767..152Z
- Keywords:
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- accretion;
- accretion disks;
- radiation mechanisms: non-thermal;
- techniques: interferometric;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 12 pages, 5 figures, 2 tables, submitted to ApJ