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

Detection of the little and big blue bumps

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 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.