Multifrequency Studies of Active Galactic Nuclei in the Planck Satellite Era

The multi-epoch single-survey Planck satellite data have given a rare glimpse into how the radio spectra of active galactic nuclei (AGN) evolve in time. Using Planck and simultaneous auxiliary radio data ranging from 1 GHz to 857 GHz, spectra for 104 bright northern extragalactic radio sources (most of them AGN) have been assembled; in these, the various stages of flare development can be identified. The results are compared with theoretical models describing relativistic jets. Evidence for particularly flat high-frequency radio spectra is found, indicating a harder accelerated electron energy spectrum than usually assumed. A set of sources also shows signs of intrinsic cold dust.