A&A 492, 389-400 (2008)
DOI: 10.1051/0004-6361:200810937

Results of WEBT, VLBA and RXTE monitoring of 3C 279 during 2006–2007

V. M. Larionov^{1, 2}, S. G. Jorstad^{1, 3}, A. P. Marscher³, C. M. Raiteri⁴, M. Villata⁴, I. Agudo⁵, M. F. Aller⁶, A. A. Arkharov², I. M. Asfandiyarov⁷, U. Bach⁸, R. Bachev⁹, A. Berdyugin¹⁰, M. Böttcher¹¹, C. S. Buemi¹², P. Calcidese¹³, D. Carosati¹⁴, P. Charlot¹⁵, W.-P. Chen¹⁶, A. Di Paola¹⁷, M. Dolci¹⁸, S. Dogru¹⁹, V. T. Doroshenko^{20, 21, 22}, Yu. S. Efimov²⁰, A. Erdem¹⁹, A. Frasca¹², L. Fuhrmann⁸, P. Giommi²³, L. Glowienka²⁴, A. C. Gupta^{25, 26}, M. A. Gurwell²⁷, V. A. Hagen-Thorn¹, W.-S. Hsiao¹⁶, M. A. Ibrahimov⁷, B. Jordan²¹, M. Kamada²⁸, T. S. Konstantinova¹, E. N. Kopatskaya¹, Y. Y. Kovalev^{8, 29}, Y. A. Kovalev²⁹, O. M. Kurtanidze³⁰, A. Lähteenmäki³¹, L. Lanteri⁴, L. V. Larionova¹, P. Leto³², P. Le Campion¹⁵, C.-U. Lee³³, E. Lindfors¹⁰, E. Marilli¹², I. McHardy³⁴, M. G. Mingaliev³⁵, S. V. Nazarov²⁰, E. Nieppola³¹, K. Nilsson¹⁰, J. Ohlert³⁶, M. Pasanen¹⁰, D. Porter³⁷, T. Pursimo³⁸, J. A. Ros³⁹, K. Sadakane²⁸, A. C. Sadun⁴⁰, S. G. Sergeev^{20, 22}, N. Smith⁴¹, A. Strigachev⁹, N. Sumitomo²⁸, L. O. Takalo¹⁰, K. Tanaka²⁸, C. Trigilio¹², G. Umana¹², H. Ungerechts⁴², A. Volvach⁴³, and W. Yuan<sup>25

1</sup>  Astron. Inst., St.-Petersburg State Univ., Russia
    e-mail: vlar@astro.spbu.ru

²  Pulkovo Observatory, St.-Petersburg, Russia

³  Inst. for Astrophys. Research, Boston Univ., MA, USA

⁴  INAF, Osservatorio Astronomico di Torino, Italy 

⁵  Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain

⁶  Department of Astronomy, University of Michigan, MI, USA

⁷  Ulugh Beg Astron. Inst., Tashkent, Uzbekistan

⁸  Max-Planck-Institut für Radioastronomie, Bonn, Germany

⁹  Inst. of Astron., Bulgarian Acad. of Sciences, Sofia, Bulgaria

¹⁰  Tuorla Observatory, Univ. of Turku, Piikkiö, Finland

¹¹  Department of Physics and Astronomy, Ohio Univ., OH, USA

¹²  INAF, Osservatorio Astrofisico di Catania, Italy

¹³  Oss. Astronomico della Regione Autonoma Valle d'Aosta, Italy

¹⁴  Armenzano Astronomical Observatory, Italy 

¹⁵  Lab. d'Astrophys., Univ. Bordeaux 1, CNRS, Floirac, France

¹⁶  Institute of Astronomy, National Central University, Taiwan

¹⁷  INAF, Osservatorio Astronomico di Roma, Italy

¹⁸  INAF, Osservatorio Astronomico di Collurania Teramo, Italy

¹⁹  COMU Observatory, Turkey

²⁰  Crimean Astrophysical Observatory, Ukraine 

²¹  Moscow Univ., Crimean Lab. of Sternberg Astron. Inst., Ukraine

²²  Isaac Newton Institute of Chile, Crimean Branch, Ukraine

²³  ASI Science Data Centre, Frascati, Italy

²⁴  Department of Phys. and Astron.  Univ. of Aarhus, Denmark 

²⁵  YNAO, Chinese Academy of Sciences, Kunming, PR China

²⁶  ARIES, Manora Peak, Nainital, India
²⁷  Harvard-Smithsonian Center for Astroph., Cambridge, MA, USA

²⁸  Astronomical Institute, Osaka Kyoiku University, Japan

²⁹  Astro Space Centre of Lebedev Physical Inst., Moscow, Russia

³⁰  Abastumani Astrophysical Observatory, Georgia

³¹  Metsähovi Radio Obs., Helsinki Univ. of Technology, Finland

³²  INAF, Istituto di Radioastronomia, Sezione di Noto, Italy

³³  Korea Astronomy and Space Science Institute, South Korea

³⁴  University of Southampton, UK
³⁵  Special Astrophysical Observatory, N. Arkhyz, Russia

³⁶  Michael Adrian Observatory, Trebur, Germany 

³⁷  Cardiff University, Wales, UK

³⁸  Nordic Optical Telescope, Santa Cruz de La Palma, Spain

³⁹  Agrupació Astronòmica de Sabadell, Spain

⁴⁰  Dept. of Phys., Univ. of Colorado, Denver, USA

⁴¹  Cork Institute of Technology, Cork, Ireland

⁴²  Instituto de Radioastronomía Milimétrica, Granada, Spain

⁴³  Radio Astron. Lab. of Crimean Astroph. Observatory, Ukraine


Received 8 September 2008 / Accepted 17 October 2008

Abstract
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.

Key words: galaxies: active -- quasars: general -- quasars: individual: 3C 279

© ESO 2008

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