Multiwavelength observations of 3C 454.3 II. The AGILE 2007 December
  campaign
  We report on the second AGILE multiwavelength campaign of the blazar 3C 454.3
during the first half of December 2007. This campaign involved AGILE, Spitzer,
Swift,Suzaku,the WEBT consortium,the REM and MITSuME telescopes,offering a
broad band coverage that allowed for a simultaneous sampling of the synchrotron
and inverse Compton (IC) emissions.The 2-week AGILE monitoring was accompanied
by radio to optical monitoring by WEBT and REM and by sparse observations in
mid-Infrared and soft/hard X-ray energy bands performed by means of Target of
Opportunity observations by Spitzer, Swift and Suzaku, respectively.The source
was detected with an average flux of~250x10^{-8}ph cm^-2s^-1 above 100
MeV,typical of its flaring states.The simultaneous optical and gamma-ray
monitoring allowed us to study the time-lag associated with the variability in
the two energy bands, resulting in a possible ~1-day delay of the gamma-ray
emission with respect to the optical one. From the simultaneous optical and
gamma-ray fast flare detected on December 12, we can constrain the delay
between the gamma-ray and optical emissions within 12 hours. Moreover, we
obtain three Spectral Energy Distributions (SEDs) with simultaneous data for
2007 December 5, 13, 15, characterized by the widest multifrequency coverage.
We found that a model with an external Compton on seed photons by a standard
disk and reprocessed by the Broad Line Regions does not describe in a
satisfactory way the SEDs of 2007 December 5, 13 and 15. An additional
contribution, possibly from the hot corona with T=10^6 K surrounding the jet,
is required to account simultaneously for the softness of the synchrotron and
the hardness of the inverse Compton emissions during those epochs.
