  We report the results of a 3-weeks multi-wavelength campaign on the flat
spectrum radio quasar 3C 273 carried out with the AGILE gamma-ray mission,
covering the 30 MeV -50 GeV and 18-60 keV, the REM observatory (covering the
near-IR and optical), Swift (near-UV/Optical, 0.2-10 keV and 15-50 keV),
INTEGRAL (3 - 200 keV) and Rossi XTE (2-12 keV). This is the first
observational campaign including gamma-ray data, after the last EGRET
observations, more than 8 years ago. Our study was carried out using
simultaneous light curves of the source flux from all the involved instruments,
in the different energy ranges, in search for correlated variability. Then a
time-resolved SED was used for a detailed physical modelling of the emission
mechanisms. The source was detected in gamma-rays only in the second week of
our campaign. We found indication of a possible anti-correlation between the
emission at gamma-rays and at soft and hard X-rays, supported by the complete
set of instruments. Instead, optical data do not show short term variability.
Only in two EGRET observations (in 1993 and 1997) 3C 273 showed
intra-observation variability in gamma-rays. The SED with almost simultaneous
data, partially covers the regions of the synchrotron emission, the big blue
bump, and the inverse-Compton. The soft X-ray emission is consistent with
combined SSC and EC mechanisms, while hard X and gamma-ray emissions are
compatible with EC from thermal photons of the disk. The time evolution of the
SED is well interpreted and modelled in terms of an acceleration episode of the
electrons population, leading to a shift in the IC peak towards higher
energies.
