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

Detection of the little and big blue bumps

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)

Abstract
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.