EDP Sciences
Free access
Volume 473, Number 3, October III 2007
Page(s) 819 - 827
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20078289

A&A 473, 819-827 (2007)
DOI: 10.1051/0004-6361:20078289

WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase

Detection of the little and big blue bumps
C. M. Raiteri1, M. Villata1, V. M. Larionov2, 3, T. Pursimo4, M. A. Ibrahimov5, K. Nilsson6, M. F. Aller7, O. M. Kurtanidze8, L. Foschini9, J. Ohlert10, I. E. Papadakis11, 12, N. Sumitomo13, A. Volvach14, H. D. Aller7, A. A. Arkharov3, U. Bach15, A. Berdyugin6, M. Böttcher16, C. S. Buemi17, P. Calcidese18, P. Charlot19, A. J. Delgado Sánchez20, A. Di Paola21, A. A. Djupvik4, M. Dolci22, N. V. Efimova3, J. H. Fan23, E. Forné24, C. A. Gomez2, A. C. Gupta25, V. A. Hagen-Thorn2, L. Hooks16, T. Hovatta26, Y. Ishii13, M. Kamada13, T. Konstantinova2, E. Kopatskaya2, Yu. A. Kovalev27, Y. Y. Kovalev15, 27, A. Lähteenmäki26, L. Lanteri1, J.-F. Le Campion19, C.-U. Lee28, P. Leto29, H.-C. Lin30, E. Lindfors6, M. G. Mingaliev31, S. Mizoguchi13, F. Nicastro21, M. G. Nikolashvili8, S. Nishiyama13, L. Östman32, E. Ovcharov33, P. Pääkkönen34, M. Pasanen6, E. Pian35, T. Rector36, J. A. Ros24, K. Sadakane13, J. H. Selj37, E. Semkov38, D. Sharapov4, A. Somero4, 39, I. Stanev33, A. Strigachev38, L. Takalo6, K. Tanaka13, M. Tavani40, I. Torniainen26, M. Tornikoski26, C. Trigilio17, G. Umana17, S. Vercellone41, A. Valcheva33, 38, L. Volvach14, and M. Yamanaka13


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  Nordic Optical Telescope, Santa Cruz de La Palma, Spain

5  Ulugh Beg Astron. Inst., Academy of Sciences of Uzbekistan, Tashkent, Uzbekistan

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

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

8  Abastumani Astrophysical Observatory, Georgia

9  INAF, IASF-Bologna, Italy

10  Michael Adrian Observatory, Trebur, Germany

11  IESL, FORTH, Heraklion, Crete, Greece

12  Physics Department, University of Crete, Greece

13  Astronomical Institute, Osaka Kyoiku University, Japan

14  Radio Astronomy Lab. of Crimean Astrophysical Observatory, Ukraine

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

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

17  INAF, Osservatorio Astrofisico di Catania, Italy

18  Osservatorio Astronomico della Regione Autonoma Valle d'Aosta, Italy

19  Université Bordeaux 1/OASU - CNRS/UMR 5804, France

20  Instituto de Astrofísica de Andalucía, Granada, Spain

21  INAF, Osservatorio Astronomico di Roma, Italy

22  INAF, Osservatorio Astronomico di Collurania Teramo, Italy

23  Center for Astrophysics, Guangzhou University, PR China

24  Agrupació Astronòmica de Sabadell, Spain

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

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

27  Astro Space Center of Lebedev Physical Inst., Moscow, Russia

28  Korea Astronomy and Space Science Institute, South Korea

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

30  Institute of Astronomy, National Central University, Taiwan

31  Special Astrophysical Observatory, Russia

32  Dept. of Physics, Stockholm University, Sweden

33  Sofia University, Bulgaria

34  Univ. of Joensuu, Dept. of Physics and Mathematics, Finland

35  INAF, Osservatorio Astronomico di Trieste, Italy

36  University of Alaska Anchorage, AK, USA

37  Inst. of Theoretical Astrophysics, Univ. of Oslo, Norway

38  Inst. of Astronomy, Bulgarian Academy of Sciences, Sofia, Bulgaria

39  Observatory, Univ. of Helsinki, Finland

40  INAF, IASF-Roma, Italy

41  INAF, IASF-Milano, Italy

(Received 16 July 2007 / Accepted 14 August 2007)

Context.The quasar-type blazar 3C 454.3 was observed to undergo an unprecedented optical outburst in spring 2005, affecting the source brightness from the near-IR to the X-ray frequencies. This was first followed by a millimetric and then by a radio outburst, which peaked in February 2006.
Aims.In this paper we report on follow-up observations to study the multiwavelength emission in the post-outburst phase.
Methods.Radio, near-infrared, and optical monitoring was performed by the Whole Earth Blazar Telescope (WEBT) collaboration in the 2006-2007 observing season. XMM-Newton observations on July 2-3 and December 18-19, 2006 added information on the X-ray and UV states of the source.
Results.The source was in a faint state. The radio flux at the higher frequencies showed a fast decreasing trend, which represents the tail of the big radio outburst. It was followed by a quiescent state, common at all radio frequencies. In contrast, moderate activity characterized the near-IR and optical light curves, with a progressive increase of the variability amplitude with increasing wavelength. We ascribe this redder-when-brighter behaviour to the presence of a "little blue bump" due to line emission from the broad line region, which is clearly visible in the source spectral energy distribution (SED) during faint states. Moreover, the data from the XMM-Newton Optical Monitor reveal a rise of the SED in the ultraviolet, suggesting the existence of a "big blue bump" due to thermal emission from the accretion disc. The X-ray spectra are well fitted with a power-law model with photoelectric absorption, possibly larger than the Galactic one. However, the comparison with previous X-ray observations would imply that the amount of absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum presents a curvature, which may depend on the X-ray brightness. In this case, two scenarios are possible. i) There is no extra absorption, and the X-ray spectrum hardens at low energies, the hardening being more evident in bright states; ii) there is a constant amount of extra absorption, likely in the quasar environment, and the X-ray spectrum softens at low energies, at least in faint X-ray states. This softening might be the result of a flux contribution by the high-frequency tail of the big blue bump.

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

© ESO 2007