  We study the quasi-simultaneous Spectral Energy Distributions (SED) of 48
LBAS blazars, collected within three months of the Fermi LAT Bright AGN Sample
(LBAS) data taking period, combining Fermi and Swift data with radio
NIR-Optical and hard-X/gamma-ray data. We have used these SEDs to characterize
the peak position and intensity of both the low and the high-energy features of
blazar spectra. The results have been used to derive empirical relationships
that estimate the position of the two peaks from the broad-band colors (i.e.
the radio to optical, alpha_ro, and optical to X-ray, alpha_ox, spectral
slopes) and from the gamma-ray spectral index. Our data show that the
synchrotron peak frequency is positioned between 10^(12.5) and 10^(14.5) Hz in
broad-lined FSRQs and between 10^(13) and 10^(17) Hz in featureless BL Lacertae
objects. We find that the gamma-ray spectral slope is strongly correlated with
the synchrotron peak energy and with the X-ray spectral index, as expected at
first order in synchrotron - inverse Compton scenarios. However, simple
homogeneous, one-zone, Synchrotron Self Compton (SSC) models cannot explain
most of our SEDs, especially in the case of FSRQs and low energy peaked (LBL)
BL Lacs. More complex models involving External Compton Radiation or multiple
SSC components are required to reproduce the overall SEDs and the observed
spectral variability.
