A. A. Abdo et al. (incl. M. Böttcher),
Nature, submitted (2009)
It is widely accepted that strong and variable radiation over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight. Such jet-dominated objects are commonly called ``blazars''. It is also established that their polarized radio and optical emission is due to synchrotron radiation of relativistic jet electrons, whereas gamma-rays most likely result from inverse-Compton scattering of surrounding photon fields by the same population of particles. However, the size of the emitting zone and the location of this region relative to the central supermassive black hole are poorly known, and various estimates differ by orders of magnitude. Knowing those quantities is crucial for testing the current paradigm of the jet production in active galaxies involving a strong, rotating magnetic field anchored by an accretion disk orbiting the black hole. Here, we present new broad-band observational data for the well-known blazar 3C279 from multiple observatories, including gamma-ray measurements from the recently-launched Fermi Gamma-Ray Space Telescope, optical and near-infrared photometry and polarimetry, as well as X-ray and radio-band data. The coincidence of a gamma-ray flare with the dramatic change of optical polarization angle measured by the KANATA telescope in japan provides evidence for co-spatiality of optical and gamma-ray emission regions and indicates a highly ordered jet magnetic field. In addition, it indicates non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet related to either a helical magnetic field or jet bending due to interaction with the external medium. If we are observing a single moving source throughout the variation, the ~20-day duration of the polarization event implies it is located at ~105 gravitational radii away from the central black hole; if the modulation is caused by the motion of the whole jet, the emission region may potentially originate much closer to the black hole, at ~103 gravitational radii.