  We report on the multi-wavelength observations of PKS 1510-089 (a flat
spectrum radio quasar at z=0.361) during its high activity period between 2008
September and 2009 June. During this 11 months period, the source was
characterized by a complex variability at optical, UV and gamma-ray bands, on
time scales down to 6-12 hours. The brightest gamma-ray isotropic luminosity,
recorded on 2009 March 26, was ~ 2x10^48erg s^-1. The spectrum in the Fermi-LAT
energy range shows a mild curvature well described by a log-parabolic law, and
can be understood as due to the Klein-Nishina effect. The gamma-ray flux has a
complex correlation with the other wavelengths. There is no correlation at all
with the X-ray band, a weak correlation with the UV, and a significant
correlation with the optical flux. The gamma-ray flux seems to lead the optical
one by about 13 days. From the UV photometry we estimated a black hole mass of
~ 5.4x10^8 solar masses, and an accretion rate of ~ 0.5 solar masses/year.
Although the power in the thermal and non-thermal outputs is smaller compared
to the very luminous and distant flat spectrum radio quasars, PKS 1510-089
exhibits a quite large Compton dominance and a prominent big blue bump (BBB) as
observed in the most powerful gamma-ray quasars. The BBB was still prominent
during the historical maximum optical state in 2009 May, but the optical/UV
spectral index was softer than in the quiescent state. This seems to indicate
that the BBB was not completely dominated by the synchrotron emission during
the highest optical state. We model the broadband spectrum assuming a leptonic
scenario in which the inverse Compton emission is dominated by the scattering
of soft photons produced externally to the jet. The resulting model-dependent
jet energetic content is compatible with the accretion disk powering the jet,
with a total efficiency within the Kerr black hole limit.
