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Birth of a relativistic outflow in the unusual γ-ray transient Swift J164449.3+573451


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.

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Figure 1: Radio and optical images of Swift J164449.3+573451 and its host galaxy reveal a positional alignment between the transient and the centre of the galaxy.
Figure 2: SEDs (luminosity, νL ν ; frequency, ν ) of Swift J164449.3+573451 from radio to X-rays point to synchrotron emission from a relativistic outflow.
Figure 3: Radio light curves of Swift J164449.3+573451 at 5–345 GHz reveal interstellar scintillation.


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E.B. is supported in part by funds from NASA. A.L. is supported in part by NSF and NASA grants. R.M. acknowledges support from a Swift ASI grant and from the Ministry of Universities and Research of Italy. E.N. is partly supported by IRG and ISF grants. The EVLA and the Very Long Baseline Array are operated by the NRAO, a facility of the NSF operated under cooperative agreement by the AUI. The SMA is a joint project between the SAO and the ASIAA, and is funded by the Smithsonian Institution and the Academia Sinica. CARMA development and operations are supported by the NSF under a cooperative agreement, and by the Associates of the California Institute of Technology, the University of Chicago and the states of California, Illinois and Maryland. The AMI arrays are supported by the University of Cambridge and the STFC. This work is partly based on observations with the 100-m telescope of the MPIfR at Effelsberg. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

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B.A.Z. and E.B. designed and coordinated the radio observations and analysis among all instruments reported here. B.A.Z. and D.A.F. performed EVLA observations, data reduction and analysis. G.R.P. observed the source with the SMA, and along with N.A.P. reduced and analysed the SMA observations. CARMA observations were set up, reduced and analysed by B.A.Z., J.M.C. and S.R.K. Fast-response CARMA observations were facilitated by N.H.V. and T.L.C., with the first CARMA observation made by S.S. and C.L.H.H. R.C. implemented and analysed MMT and Gemini optical observations. K.M. performed observations with the OVRO 40-m telescope and analysed results, with advice from S.R.K., A.C.S.R., J.R., M.S. and W.M-M. G.G.P. performed observations with the AMI Large Array and analysed the results. A.M.S., A.B., M.F.B. and M.P.R. planned observations with the VLBA and the MPIfR. A.B. reduced the VLBI data. R.M. analysed and modelled the X-ray data. A.L., R.N. and E.N. provided the theoretical model for a tidal disruption event. The paper was put together by B.A.Z. and E.B., with the primary text written by E.B. and portions of Supplementary Information written by E.B., B.A.Z., R.C., K.M. and A.B. D.B.F. provided feedback on the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to B. A. Zauderer or E. Berger.

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The authors declare no competing financial interests.

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The file contains Supplementary Text and Data 1-9, Supplementary Tables 1-2, Supplementary Figures 1-2 with legends and additional references. (PDF 381 kb)

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Zauderer, B., Berger, E., Soderberg, A. et al. Birth of a relativistic outflow in the unusual γ-ray transient Swift J164449.3+573451. Nature 476, 425–428 (2011).

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