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Astrophysics and Astronomy

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Published in the journal Astronomy & Astrophysics in the section Extragalactic Astronomy.

Larionov, V.M. et al., A&A, 492.,389-400, 2008, reproduced with permission, © ESO

The research at Boston University was funded in part by the National Science Foundation through grant AST-0406865 and by NASA through RXTE Guest Investigator grant NNX06AG86G, and Astrophysical Data Analysis Program grant NNX08AJ64G. The VLBA is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation, USA, operated under cooperative agreement by Associated Universities, Inc. This work is partly based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Partly based on observations with the Medicina and Noto telescopes operated by INAF – Istituto di Radioastronomia. This research has made use of data from the University of Michigan Radio Astronomy Observatory, which is supported by the National Science Foundation and by funds from the University of Michigan. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. The Metsähovi team acknowledges the support from the Academy of Finland. AZT-24 observations are made within an agreement between Pulkovo, Rome and Teramo observatories. The Torino team acknowledges financial support by the Italian Space Agency through contract ASI-INAF I/088/06/0 for the Study of HighEnergy Astrophysics. Y. Y. Kovalev is a Research Fellow of the Alexander von Humboldt Foundation. RATAN–600 observations are partly supported by the Russian Foundation for Basic Research (projects 01-02-16812, 05-02-17377, 08-02-00545). We thank Tuomas Savolainen for useful discussion. This paper is partly based on observations carried out at the 30-m telescope of IRAM, which is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). I.A. acknowledges support by the CSIC through an I3P contract, and by the “Ministerio de Ciencia e Innovación” and the European Fund for Regional Development through grant AYA2007-67627-C03-03. ACG’s and WY’s work is supported by NNSF of China grant No. 10533050.


Context. The quasar 3C 279 is among the most extreme blazars in terms of luminosity and variability of flux at all wavebands. Its variations in flux and polarization are quite complex and therefore require intensive monitoring observations at multiple wavebands to characterise and interpret the observed changes.

Aims. In this paper, we present radio-to-optical data taken by the WEBT, supplemented by our VLBA and RXTE observations, of 3C 279. Our goal is to use this extensive database to draw inferences regarding the physics of the relativistic jet.

Methods. We assemble multifrequency light curves with data from 30 ground-based observatories and the space-based instruments SWIFT (UVOT) and RXTE, along with linear polarization vs. time in the optical R band. In addition, we present a sequence of 22 images (with polarization vectors) at 43 GHz at resolution 0.15 milliarcsec, obtained with the VLBA. We analyse the light curves and polarization, as well as the spectral energy distributions at different epochs, corresponding to different brightness states.

Results. We find that the IR-optical-UV continuum spectrum of the variable component corresponds to a power law with a constant slope of -1.6, while in the 2.4–10 keV X-ray band it varies in slope from -1.1 to -1.6. The steepest X-ray spectrum occurs at a flux minimum. During a decline in flux from maximum in late 2006, the optical and 43 GHz core polarization vectors rotate by ~300°.

Conclusions. The continuum spectrum agrees with steady injection of relativistic electrons with a power-law energy distribution of slope -3.2 that is steepened to -4.2 at high energies by radiative losses. The X-ray emission at flux minimum comes most likely from a new component that starts in an upstream section of the jet where inverse Compton scattering of seed photons from outside the jet is important. The rotation of the polarization vector implies that the jet contains a helical magnetic field that extends ~20 pc past the 43 GHz core.