C. M. Raiteri, et al. (incl. M. Böttcher and L. Hooks),
Astronomy & Astrophysics, 473, 819 (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 e nergy 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.