Multiwavelength Observations of 3C 454.3. I. The AGILE 2007 November campaign on the "Crazy Diamond"

We report on a multiwavelength observation of the blazar 3C 454.3 (which we dubbed crazy diamond) carried out on November 2007 by means of the astrophysical satellites AGILE, International Gamma-Ray Astrophysics Laboratory (INTEGRAL), Swift, the Whole Earth Blazar Telescope (WEBT) Consortium, and the optical-NIR telescope Rapid Eye Mount (REM). Thanks to the wide field of view of the AGILE satellite and its prompt alert dissemination to other observatories, we obtained a long (three weeks), almost continuous γ-ray coverage of the blazar 3C 454.3 across 14 decades of energy. This broadband monitoring allows us to study in great detail light curves, correlations, time lags, and spectral energy distributions (SEDs) during different physical states. Gamma-ray data were collected during an AGILE pointing toward the Cygnus Region. Target of Opportunity (ToO) observations were performed to follow up the γ-ray observations in the soft and hard X-ray energy bands. Optical data were acquired continuously by means of a preplanned WEBT campaign and through an REM ToO repointing. 3C 454.3 is detected at a ~19σ level during the three-week observing period, with an average flux above 100 MeV of F _{E > 100 MeV} = (170 ± 13) × 10^-8 photons cm^-2 s^-1. The γ-ray spectrum can be fitted with a single power law with photon index Γ_GRID = 1.73 ± 0.16 between 100 MeV and 1 GeV. We detect significant day-by-day variability of the γ-ray emission during our observations, and we can exclude that the fluxes are constant at the 99.6% (~2.9σ) level. The source was detected typically around 40 deg off-axis, and it was substantially off-axis in the field of view of the AGILE hard X-ray imager. However, a five-day long ToO observation by INTEGRAL detected 3C 454.3 at an average flux of about F _{20-200 keV} = 1.49 × 10^-3 photons cm^-2 s^-1with an average photon index of Γ_IBIS = 1.75 ± 0.24 between 20-200 keV. Swift also detected 3C 454.3 with a flux in the 0.3-10 keV energy band in the range (1.23-1.40) × 10^-2 photons cm^-2 s^-1 and a photon index in the range Γ_XRT = 1.56-1.73. In the optical band, both WEBT and REM show an extremely variable behavior in the R band. A correlation analysis based on the entire data set is consistent with no time lags between the γ-ray and the optical flux variations. Our simultaneous multifrequency observations strongly indicate that the dominant emission mechanism between 30 MeV and 30 GeV is dominated by inverse Compton scattering of relativistic electrons in the jet on the external photons from the broad line region.