EDP Sciences
Free access
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. Larionov2, 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. Papadakis15, 16, I. Agudo17, H. D. Aller7, E. Angelakis18, A. A. Arkharov3, U. Bach18, R. Bachev19, A. Berdyugin13, C. S. Buemi20, D. Carosati21, P. Charlot22, 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 Campion22, 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. Wiesemeyer41

1  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

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$\alpha$ emission line ( $EW_{\rm obs}$ = 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