Zauderer, BBerger, EdoSoderberg, AliciaLoeb, AbrahamNarayan, RameshFrail, D. A.Petitpas, GlenBrunthaler, A.Chornock, RCarpenter, J. M.Pooley, G. G.Mooley, K.Kulkarni, S. R.Margutti, RaffaellaFox, D. B.Nakar, E.Patel, NimeshVolgenau, N. H.Culverhouse, T. L.Bietenholz, M. F.Rupen, M. P.Max-Moerbeck, W.Readhead, A. C. S.Richards, J.Shepherd, M.Storm, S.Hull, Charles2017-10-262011Zauderer, B. A., E. Berger, A. M. Soderberg, A. Loeb, R. Narayan, D. A. Frail, G. R. Petitpas, et al. 2011. Birth of a Relativistic Outflow in the Unusual γ-Ray Transient Swift J164449.3+573451. Nature 476, no. 7361: 425–428. doi:10.1038/nature103660028-08361476-4687http://nrs.harvard.edu/urn-3:HUL.InstRepos:34299213Active 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.en-USAstronomyAstrophysicsPhysicsJournal Article2017-10-2610.1038/nature10366