Volume 473, Number 3, October III 2007
|Page(s)||819 - 827|
|Published online||28 August 2007|
Detection of the little and big blue bumps
INAF, Osservatorio Astronomico di Torino, Italy e-mail: firstname.lastname@example.org
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
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
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