We collect data at well‐sampled frequencies from the radio to the γ‐ray range for the following three complete samples of blazars: the Slew survey, the 1‐Jy samples of BL Lacs and the 2‐Jy sample of flat‐spectrum radio‐loud quasars (FSRQs). The fraction of objects detected in γ‐rays (E ≳ 100 MeV) is ∼ 17, 26 and 40 per cent in the three samples respectively. Except for the Slew survey sample, γ‐ray detected sources do not differ either from other sources in each sample, or from all the γ‐ray detected sources, in terms of the distributions of redshift, radio and X‐ray luminosities or of the broad‐band spectral indices (radio to optical and radio to X‐ray). We compute average spectral energy distributions (SEDs) from radio to γ‐rays for each complete sample and for groups of blazars binned according to radio luminosity, irrespective of the original classification as BL Lac or FSRQ. The resulting SEDs show a remarkable continuity in that (i) the first peak occurs in different frequency ranges for different samples/luminosity classes, with most luminous sources peaking at lower frequencies; (ii) the peak frequency of the γ‐ray component correlates with the peak frequency of the lower energy one; (iii) the luminosity ratio between the high and low frequency components increases with bolometric luminosity. The continuity of properties among different classes of sources and the systematic trends of the SEDs as a function of luminosity favour a unified view of the blazar phenomenon: a single parameter, related to luminosity, seems to govern the physical properties and radiation mechanisms in the relativistic jets present in BL Lac objects as well as in FSRQs. The general implications of this unified scheme are discussed while a detailed theoretical analysis, based on fitting continuum models to the individual spectra of most γ‐ray blazars, is presented in a separate paper.

The phenomenology of gamma-ray bright blazars can be accounted for by a sequence in the source power and intensity of the diffuse radiation field surrounding the relativistic jet. Correspondingly, the equilibrium particle distribution peaks at different energies. This leads to a trend in the observed properties: an increase of the observed power corresponds to: 1) a decrease in the frequencies of the synchrotron and inverse Compton peaks; 2) an increase in the ratio of the powers of the high and low energy spectral components. Objects along this sequence would be observationally classified respectively as high frequency BL Lac objects, low frequency BL Lac objects, highly polarized quasars and lowly polarized quasars. The proposed scheme is based on the correlations among the physical parameters derived in the present paper by applying to 51 gamma ray loud blazars two of the most accepted scenarios for the broad band emission of blazars, namely the synchrotron self--Compton and external Compton models, and explains the observational trends presented by Fossati et al. (1998) in a companion paper, dealing with the spectral energy distributions of all blazars. This gives us confidence that our scheme applies to all blazars as a class.Comment: 25 pages, 11 figures, uses mn.sty and psfig.tex. Accepted for publication in MNRA

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