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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., Astronomy, Accepted Manuscript

We present continued radio observations of the tidal disruption event Swift J164449.3+573451 extending to δt ≈ 216 days after discovery. The data were obtained with the EVLA, AMI Large Array, CARMA, the SMA, and the VLBA+Effelsberg as part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and to trace the parsec-scale environment around a previously-dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at δt ≈ 1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SMBH of ρ ∝ r −3/2 (0.1 − 1.2 pc) with a significant flattening at r ≈ 0.4 − 0.6 pc. The increase in energy cannot be explained with continuous injection from an L ∝ t−5/3 tail, which is observed in the X-rays. Instead, we conclude that the relativistic jet was launched with a wide range of Lorentz factors, obeying E(> Γj) ∝ Γ−2.5j. The similar ratio of duration to dynamical timescale for Sw 1644+57 and GRBs suggests that this result may be applicable to GRB jets as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at r ∼ 0.5 pc in good agreement with the Bondi radius for a ∼ few × 106 M⊙ black hole. The density at ∼ 0.5 pc is about a factor of 30 times lower than inferred for the Milky Way galactic center, potentially due to a smaller number of mass-shedding massive stars. From our latest observations (δt ≈ 216 d) we find that the jet energy is Ej,iso ≈ 5×1053 erg (Ej ≈ 2.4×1051 erg for θj = 0.1), the radius is r ≈ 1.2 pc, the Lorentz factor is Γj ≈ 2.2, the ambient density is n ≈ 0.2 cm−3, and the projected angular size is rproj ≈ 25 µas, below the resolution of the VLBA+Effelsberg. Assuming no future changes in the observed evolution and a final integrated total energy of Ej ≈ 1052 erg, we predict that the radio emission from Sw 1644+57 should be detectable with the EVLA for several decades, and will be resolvable with VLBI in a few years., Astronomy, Accepted Manuscript

Active galactic nuclei, which are powered by long-term accretion onto central supermassive black holes, produce1 relativistic jets with lifetimes of at least one million years, and the observation of the birth of such a jet is therefore unlikely. Transient accretion onto a supermassive black hole, for example through the tidal disruption2, 3 of a stray star, thus offers a rare opportunity to study the birth of a relativistic jet. On 25 March 2011, an unusual transient source (Swift J164449.3+573451) was found4, potentially representing5, 6 such an accretion event. Here we report observations spanning centimetre to millimetre wavelengths and covering the first month of evolution of a luminous radio transient associated with Swift J164449.3+573451. The radio transient coincides7 with the nucleus of an inactive galaxy. We conclude that we are seeing a newly formed relativistic outflow, launched by transient accretion onto a million-solar-mass black hole. A relativistic outflow is not predicted in this situation, but we show that the tidal disruption of a star naturally explains the observed high-energy properties and radio luminosity and the inferred rate of such events. The weaker beaming in the radio-frequency spectrum relative to γ-rays or X-rays suggests that radio searches may uncover similar events out to redshifts of z ≈ 6., Astronomy, Accepted Manuscript