Multiwavelength observations of 3C 454.3. III. Eighteen months of AGILE monitoring of the "crazy diamond"

We report on 18 months of multiwavelength observations of the blazar 3C 454.3 ( Crazy Diamond) carried out in the period 2007 July-2009 January. In particular, we show the results of the AGILE campaigns which took place on 2008 May-June, 2008 July-August, and 2008 October-2009 January. During the 2008 May-2009 January period, the source average flux was highly variable, with a clear fading trend toward the end of the period, from an average. gamma-ray flux F(E>100) (MeV) greater than or similar to 200 x 10(-8) photons cm(-2) s(-1) in 2008 May-June, to F(E>100 MeV) similar to 80 x 10(-8) photons cm(-2) s(-1) in 2008 October-2009 January. The average gamma-ray spectrum between 100 MeV and 1 GeV can be fit by a simple power law, showing a moderate softening ( from Gamma(GRID) similar to 2.0 to Gamma(GRID) similar to 2.2) toward the end of the observing campaign. Only 3 sigma upper limits can be derived in the 20-60 keV energy band with Super-AGILE, because the source was considerably off-axis during the whole time period. In 2007 July-August and 2008 May-June, 3C 454.3 was monitored by Rossi X-ray Timing Explorer (RXTE). The RXTE/Proportional Counter Array (PCA) light curve in the 3-20 keV energy band shows variability correlated with the gamma-ray one. The RXTE/PCA average flux during the two time periods is F(3-20 keV) = 8.4 x 10(-11) erg cm(-2) s(-1), and F(3-20 keV) = 4.5 x 10(-11) erg cm(-2) s(-1), respectively, while the spectrum ( a power law with photon index Gamma(PCA) = 1.65 +/- 0.02) does not show any significant variability. Consistent results are obtained with the analysis of the RXTE/High-Energy X-Ray Timing Experiment quasi-simultaneous data. We also carried out simultaneous Swift observations during all AGILE campaigns. Swift/XRT detected 3C 454.3 with an observed flux in the 2-10 keV energy band in the range (0.9-7.5) x 10(-11) erg cm(-2) s(-1) and a photon index in the range Gamma(XRT) = 1.33-2.04. In the 15-150 keV energy band, when detected, the source has an average flux of about 5 mCrab. GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period in the radio, millimeter, near-IR, and optical bands. The observations show an extremely variable behavior at all frequencies, with flux peaks almost simultaneous with those at higher energies. A correlation analysis between the optical and the gamma-ray fluxes shows that the gamma-optical correlation occurs with a time lag of tau = - 0.4(-0.8)(+0.6) days, consistent with previous findings for this source. An analysis of 15 GHz and 43 GHz VLBI core radio flux observations in the period 2007 July-2009 February shows an increasing trend of the core radio flux, anti-correlated with the higher frequency data, allowing us to derive the value of the source magnetic field. Finally, the modeling of the broadband spectral energy distributions for the still unpublished data, and the behavior of the long-term light curves in different energy bands, allow us to compare the jet properties during different emission states, and to study the geometrical properties of the jet on a time-span longer than one year.