Abstract:
Aims. The Whole Earth Blazar Telescope (WEBT) consortium has been monitoring the blazar 3C 454.3 from the radio to the optical bands since
2004 to study its emission variability properties.
Methods. We present and analyse the multifrequency results of the 2007−2008 observing season, including XMM-Newton observations and
near-IR spectroscopic monitoring, and compare the recent emission behaviour with the past one. The historical mm light curve is presented here
for the first time.
Results. In the optical band we observed a multi-peak outburst in July−August 2007, and other faster events in November 2007−February 2008.
During these outburst phases, several episodes of intranight variability were detected. A mm outburst was observed starting from mid 2007,
whose rising phase was contemporaneous to the optical brightening. A slower flux increase also affected the higher radio frequencies, the flux
enhancement disappearing below 8 GHz. The analysis of the optical-radio correlation and time delays, as well as the behaviour of the mm light
curve, confirm our previous predictions, suggesting that changes in the jet orientation likely occurred in the last few years. The historical multi-
wavelength behaviour indicates that a significant variation in the viewing angle may have happened around year 2000. Colour analysis confirms a
general redder-when-brighter trend, which reaches a “saturation” at R ∼ 14 and possibly turns into a bluer-when-brighter trend in bright states. This
behaviour is due to the interplay of different emission components, the synchrotron one possibly being characterised by an intrinsically variable
spectrum. We show the broad-band SEDs corresponding to the epochs of the XMM-Newton pointings
and compare them to those obtained at other epochs, when the source was in different brightness states. A double power-law fit to the EPIC spectra
including extra absorption suggests that the soft-X-ray spectrum is concave, and that the curvature becomes more pronounced as the flux decreases.
This connects fairly well with the UV excess, which becomes more prominent with decreasing flux. The most obvious interpretation implies that,
as the beamed synchrotron radiation from the jet dims, we can see both the head and the tail of the big blue bump. The X-ray flux correlates with
the optical flux, suggesting that in the inverse-Compton process either the seed photons are synchrotron photons at IR-optical frequencies or the
relativistic electrons are those that produce the optical synchrotron emission. The X-ray radiation would thus be produced in the jet region from
where the IR-optical emission comes.
