The December 2009 gamma-ray flare of 3C 454.3: the multifrequency
  campaign
  During the month of December, 2009 the blazar 3C 454.3 became the brightest
gamma-ray source in the sky, reaching a peak flux F ~2000E-8 ph/cm2/s for E >
100 MeV. Starting in November, 2009 intensive multifrequency campaigns
monitored the 3C 454 gamma-ray outburst. Here we report the results of a
2-month campaign involving AGILE, INTEGRAL, Swift/XRT, Swift/BAT, RossiXTE for
the high-energy observations, and Swift/UVOT, KANATA, GRT, REM for the
near-IR/optical/UV data. The GASP/WEBT provided radio and additional optical
data. We detected a long-term active emission phase lasting ~1 month at all
wavelengths: in the gamma-ray band, peak emission was reached on December 2-3,
2009. Remarkably, this gamma-ray super-flare was not accompanied by
correspondingly intense emission in the optical/UV band that reached a level
substantially lower than the previous observations in 2007-2008. The lack of
strong simultaneous optical brightening during the super-flare and the
determination of the broad-band spectral evolution severely constrain the
theoretical modelling. We find that the pre- and post-flare broad-band behavior
can be explained by a one-zone model involving SSC plus external Compton
emission from an accretion disk and a broad-line region. However, the spectra
of the Dec. 2-3, 2009 super-flare and of the secondary peak emission on Dec. 9,
2009 cannot be satisfactorily modelled by a simple one-zone model. An
additional particle component is most likely active during these states.
