The inner jet of an active galactic nucleus as revealed by a radio-to-γ-ray outburst


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 ergosphere1,2,3. The flow velocity increases outward along the jet in an acceleration and collimation zone containing a coiled magnetic field4,5. 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 γ-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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Figure 1: Sequence of Very Long Baseline Array images of BL Lac at a wavelength of 7 mm (and a frequency of 43 GHz).
Figure 2: Flux density at various wavebands and optical polarization of BL Lac, as functions of time.
Figure 3: Proposed model for the inner jet of BL Lac.


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The authors thank N. Vlahakis for discussions. The research reported here is based on work supported in part by the US National Science Foundation, NASA, the Russian Fund for Basic Research, the Academy of Finland, and the Italian Space Agency (ASI). The Very Long Baseline Array is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Correspondence to Alan P. Marscher.

Supplementary information

Supplementary information

The file contains Supplementary Discussion, supplementary Methods and additional references. The Supplementary Discussion presents calculations that relate the rate of rotation of the optical polarisation vector and apparent speed of the knot of emission to the angular velocity of the knot. This leads to an estimate of the speed of rotation and distance from the black hole of the footpoint of the jet's magnetic field in either the accretion disk or the ergosphere. The Supplementary Methods give details of the observations and data analysis are given. (PDF 283 kb)

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Marscher, A., Jorstad, S., D’Arcangelo, F. et al. The inner jet of an active galactic nucleus as revealed by a radio-to-γ-ray outburst. Nature 452, 966–969 (2008).

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