Fermi and multifrequency observations of blazars

Abstract

The Fermi Gamma‐ray Space Telescope was successfully launched on June 11, 2008 and has already opened a new era for gamma‐ray astronomy. The Large Area Telescope (LAT), the main instrument on board Fermi, with its large field of view and effective area, combined with its excellent timing capabilities, presents a significant improvement in sensitivity over its predecessor EGRET. The preliminary results of the Spectral Energy Distribution Analysis performed on a sample of bright blazars are presented. We have studied the quasi‐simultaneous Spectral Energy Distributions (SED) of 48 blazars, detected within the 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 (αro) and optical to X‐ray (αox) spectral slopes, and from the gamma‐ray spectral index. Our data show that the synchrotron peak frequency is positioned between 1012.5 and 1014.5 Hz in FSRQs and between 1013 and 1017 Hz in BL Lacertae objects. We find that the gamma‐ray spectral slope is strongly correlated with the synchrotron peak energy, 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.