Issue A&A Volume 491, Number 3, December I 2008 Page(s) 755 - 766 Section Extragalactic astronomy DOI http://dx.doi.org/10.1051/0004-6361:200810869 Published online 15 October 2008

A&A 491, 755-766 (2008)
DOI: 10.1051/0004-6361:200810869

A new activity phase of the blazar 3C 454.3

Multifrequency observations by the WEBT and XMM-Newton in 2007–2008

C. M. Raiteri¹, M. Villata¹, V. M. Larionov^{2, 3}, M. A. Gurwell⁴, W. P. Chen⁵, O. M. Kurtanidze⁶, M. F. Aller⁷, M. Böttcher⁸, P. Calcidese⁹, F. Hroch¹⁰, A. Lähteenmäki¹¹, C.-U. Lee¹², K. Nilsson¹³, J. Ohlert¹⁴, I. E. Papadakis^{15, 16}, I. Agudo¹⁷, H. D. Aller⁷, E. Angelakis¹⁸, A. A. Arkharov³, U. Bach¹⁸, R. Bachev¹⁹, A. Berdyugin¹³, C. S. Buemi²⁰, D. Carosati²¹, P. Charlot^{22, 23}, E. Chatzopoulos¹⁶, E. Forné²⁴, A. Frasca²⁰, L. Fuhrmann¹⁸, J. L. Gómez¹⁷, A. C. Gupta²⁵, V. A. Hagen-Thorn², W.-S. Hsiao⁵, B. Jordan²⁶, S. G. Jorstad²⁷, T. S. Konstantinova², E. N. Kopatskaya², T. P. Krichbaum¹⁸, L. Lanteri¹, L. V. Larionova², G. Latev²⁸, J.-F. Le Campion^{22, 23}, P. Leto²⁹, H.-C. Lin⁵, N. Marchili¹⁸, E. Marilli²⁰, A. P. Marscher²⁷, B. McBreen³⁰, B. Mihov¹⁹, R. Nesci³¹, F. Nicastro³², M. G. Nikolashvili⁶, R. Novak³³, E. Ovcharov²⁸, E. Pian³⁴, D. Principe⁸, T. Pursimo³⁵, B. Ragozzine⁸, J. A. Ros²⁴, A. C. Sadun³⁶, R. Sagar²⁵, E. Semkov¹⁹, R. L. Smart¹, N. Smith³⁷, A. Strigachev¹⁹, L. O. Takalo¹³, M. Tavani³⁸, M. Tornikoski¹¹, C. Trigilio²⁰, K. Uckert⁸, G. Umana²⁰, A. Valcheva¹⁹, S. Vercellone³⁹, A. Volvach⁴⁰, and H. Wiesemeyer<sup>41

1</sup>  INAF – Osservatorio Astronomico di Torino, Italy
    e-mail: raiteri@oato.inaf.it
²  Astron. Inst., St.-Petersburg State Univ., Russia
³  Pulkovo Observatory, St. Petersburg, Russia
⁴  Harvard-Smithsonian Center for Astroph., Cambridge, MA, USA
⁵  Institute of Astronomy, National Central University, Taiwan
⁶  Abastumani Astrophysical Observatory, Georgia
⁷  Department of Astronomy, University of Michigan, MI, USA
⁸  Department of Physics and Astronomy, Ohio Univ., OH, USA
⁹  Osservatorio Astronomico della Regione Autonoma Valle d'Aosta, Italy
¹⁰  Inst. of Theor. Phys. and Astroph., Masaryk Univ., Czech Republic
¹¹  Metsähovi Radio Obs., Helsinki Univ. of Technology, Finland
¹²  Korea Astronomy and Space Science Institute, South Korea
¹³  Tuorla Observatory, Univ. of Turku, Piikkiö, Finland
¹⁴  Michael Adrian Observatory, Trebur, Germany
¹⁵  IESL, FORTH, Heraklion, Crete, Greece
¹⁶  Physics Department, University of Crete, Greece
¹⁷  Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
¹⁸  Max-Planck-Institut für Radioastronomie, Bonn, Germany
¹⁹  Inst. of Astronomy, Bulgarian Academy of Sciences, Sofia, Bulgaria
²⁰  INAF – Osservatorio Astrofisico di Catania, Italy
²¹  Armenzano Astronomical Observatory, Italy
²²  Université de Bordeaux, Observatoire Aquitain des Sciences de l'Univers, Floirac, France
²³  CNRS, Laboratoire d'Astrophysique de Bordeaux, UMR 5804, Floirac, France
²⁴  Agrupació Astronòmica de Sabadell, Spain
²⁵  ARIES, Manora Peak, Nainital, India
²⁶  School of Cosmic Physics, Dublin Institute For Advanced Studies, Ireland
²⁷  Institute for Astrophysical Research, Boston University, MA, USA
²⁸  Sofia University, Bulgaria
²⁹  INAF – Istituto di Radioastronomia, Sezione di Noto, Italy
³⁰  School of Physics, University College Dublin, Ireland
³¹  Dept. of Phys. “La Sapienza” Univ, Roma, Italy
³²  INAF – Osservatorio Astronomico di Roma, Italy
³³  N. Copernicus Observatory and Planetarium in Brno, Czech Republic
³⁴  INAF – Osservatorio Astronomico di Trieste, Italy
³⁵  Nordic Optical Telescope, Santa Cruz de La Palma, Spain
³⁶  Dept. of Phys., Univ. of Colorado Denver, Denver, CO USA
³⁷  Cork Institute of Technology, Cork, Ireland
³⁸  INAF, IASF-Roma, Italy
³⁹  INAF, IASF-Milano, Italy
⁴⁰  Radio Astronomy Lab. of Crimean Astrophysical Observatory, Ukraine
⁴¹  Instituto de Radioastronomía Millimétrica, Granada, Spain

Received 27 August 2008 / Accepted 2 October 2008

Abstract
Aims. The Whole Earth Blazar Telescope (WEBT) consortium has been monitoring the blazar 3C 454.3 from the radio to the optical bands since 2004 to study its emission variability properties.
Methods. We present and analyse the multifrequency results of the 2007-2008 observing season, including XMM-Newton observations and near-IR spectroscopic monitoring, and compare the recent emission behaviour with the past one. The historical mm light curve is presented here for the first time.
Results. In the optical band we observed a multi-peak outburst in July-August 2007, and other faster events in November 2007-February 2008. During these outburst phases, several episodes of intranight variability were detected. A mm outburst was observed starting from mid 2007, whose rising phase was contemporaneous to the optical brightening. A slower flux increase also affected the higher radio frequencies, the flux enhancement disappearing below 8 GHz. The analysis of the optical-radio correlation and time delays, as well as the behaviour of the mm light curve, confirm our previous predictions, suggesting that changes in the jet orientation likely occurred in the last few years. The historical multiwavelength behaviour indicates that a significant variation in the viewing angle may have happened around year 2000. Colour analysis confirms a general redder-when-brighter trend, which reaches a “saturation” at R ~ 14 and possibly turns into a bluer-when-brighter trend in bright states. This behaviour is due to the interplay of different emission components, the synchrotron one possibly being characterised by an intrinsically variable spectrum. All the near-IR spectra show a prominent H emission line (  = 50-120 Å), whose flux appears nearly constant, indicating that the broad line region is not affected by the jet emission. We show the broad-band SEDs corresponding to the epochs of the XMM-Newton pointings and compare them to those obtained at other epochs, when the source was in different brightness states. A double power-law fit to the EPIC spectra including extra absorption suggests that the soft-X-ray spectrum is concave, and that the curvature becomes more pronounced as the flux decreases. This connects fairly well with the UV excess, which becomes more prominent with decreasing flux. The most obvious interpretation implies that, as the beamed synchrotron radiation from the jet dims, we can see both the head and the tail of the big blue bump. The X-ray flux correlates with the optical flux, suggesting that in the inverse-Compton process either the seed photons are synchrotron photons at IR-optical frequencies or the relativistic electrons are those that produce the optical synchrotron emission. The X-ray radiation would thus be produced in the jet region from where the IR-optical emission comes.

Key words: galaxies: active -- galaxies: quasars: general -- galaxies: quasars: individual: 3C 454.3 -- galaxies: jets

© ESO 2008

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