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 F3-20 keV = 8.4 x 10(-11) erg cm(-2) s(-1), and F3-20 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 ha