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 \sim 250 \times 10 ^ { -8 } photons cm ^ { -2 } s ^ { -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 \lesssim 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 .