The multi-frequency behaviour of Blazars

We have conducted a detailed investigation of the broad-ban d spectral properties of the gamma -ray se- lected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately es - timated Fermi gamma -ray spectra with Swift , radio, infra-red, optical and other hard X-ray/ gamma -ray data, collected within three months of the LBAS data taking period , we were able to assemble high- quality and quasi-simultaneous Spectral Energy Distribut ions (SED) for 48 LBAS blazars. The SED of these gamma -ray sources is similar to that of blazars discovered at othe r wavelengths, clearly showing, in the usual Log nu – Log nu F nu representation, the typical broad-band spectral signatur es normally attributed to a combination of low-energy synchro tron radiation followed by inverse Compton emission of one or more components. We have used thes e SEDs to characterize the peak intensity of both the low and the high-energy component s. 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. We find that the gamma -ray spectral slope is strongly correlated with the synchro tron 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 cas e of FSRQs and low energy peaked (LSP) BL Lacs. More complex models involving External Compt on Radiation or multiple SSC components are required to reproduce the overall SEDs and th e observed spectral variability. While more than 50% of known radio bright high energy peaked ( HSP, HBL in the old BL Lac nomenclature) BL Lacs are detected in the LBAS sample, only l ess than 13% of known bright FSRQs and LSP BL Lacs are included. This suggests that the lat ter sources, as a class, may be much fainter gamma -ray emitters than LBAS blazars, and could in fact radiate cl ose to the expectations of simple SSC models.