The Connection between Radio and Gamma-ray Emission in Active Galactic Nuclei

Radio and gamma-ray emission from active galactic nuclei (AGN) are thought to share a common origin, related to the ejection phenomena in the vicinity of supermassive black holes. Thanks to its sensitivity, surveying capability, and broad energy range, the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope has permitted us to discover and characterize a huge number of extragalactic gamma-ray sources. Similarly to what was found by EGRET, these sources are typically associated with blazars, characterized by significant radio emission and flat spectrum. The radio luminosity distribution is extended over 7 orders of magnitudes, with flat spectrum radio quasars clustered at higher powers and BL Lacs more scattered; the average spectral index is consistent with alpha=0, although a few remarkable sources have alpha>0.5. A comparison of the radio flux density and the gamma-ray photon flux is presented, although claims on its significance require a detailed discussion and Monte Carlo simulations which will be presented in a future paper.