1. Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA
2. Astronomical Institute of St Petersburg State University, Universitetskij Prospect 28, Petrodvorets, 198504 St Petersburg, Russia
3. Steward Observatory,
4. MMT Observatory, University of Arizona, Tucson, Arizona 85721-0065, USA
5. Astronomy Department, University of Michigan, 830 Dennison Building, Ann Arbor, Michigan 48109-1090, USA
6. Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
7. Metsähovi Radio Observatory, Helsinki University of Technology TKK, Metsähovintie 114, 02540 Kylmälä, Finland
8. Tuorla Observatory Väisäläntie 20, 21500 Piikkiö, Finland
9. Department of Physics, University of Turku, 20014 Turku, Finland
10. School of Physics and Astronomy, Cardiff University, Cardiff CF24 3YB, UK
11. Department of Physics, University of Perugia, Via A. Pascoli, 06123 Perugia, Italy
12. Abastumani Astrophysical Observatory, Mt Kanobili, Abastumani, Georgia
13. Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA
14. Present address: Department of Physics, University of California, Berkeley, California 94720-7300, USA.

Correspondence to: Alan P. Marscher¹ Correspondence and requests for materials should be addressed to A.P.M. (Email: marscher@bu.edu).

Blazars are the most extreme active galactic nuclei. They possess oppositely directed plasma jets emanating at near light speeds from accreting supermassive black holes. According to theoretical models, such jets are propelled by magnetic fields twisted by differential rotation of the black hole's accretion disk or inertial-frame-dragging ergosphere^{1, 2, 3}. The flow velocity increases outward along the jet in an acceleration and collimation zone containing a coiled magnetic field^{4, 5}. Detailed observations of outbursts of electromagnetic radiation, for which blazars are famous, can potentially probe the zone. It has hitherto not been possible to either specify the location of the outbursts or verify the general picture of jet formation. Here we report sequences of high-resolution radio images and optical polarization measurements of the blazar BL Lacertae. The data reveal a bright feature in the jet that causes a double flare of radiation from optical frequencies to TeV -ray energies, as well as a delayed outburst at radio wavelengths. We conclude that the event starts in a region with a helical magnetic field that we identify with the acceleration and collimation zone predicted by the theories. The feature brightens again when it crosses a standing shock wave corresponding to the bright 'core' seen on the images.

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