Abstract

New results from multi-frequency monitoring campaigns of variable flat-spectrum radio sources are reported. They strengthen the assumption that the intraday variability occurs in a correlated fashion throughout the radio, optical and X-ray wavebands. Various properties of the behaviour exclude propagation effects as the dominant cause of the variations. This implies excessive brightness temperatures. A large fraction of the primary synchrotron radiation may be upscattered into the Gamma ray regime. We also discuss first results from polarization studies in different frequency regimes. At least the BL Lac object S5 0716+714 exhibits variations of the polarized flux which are correlated with variations of the total flux. Neither simple two component models nor “christmas-tree” scenarios of a large number of individual emitters seem able to explain the polarization data.

Introduction

Compact, flat-spectrum radio sources are well known to be variable at optical and radio frequencies. It was generally assumed that the typical timescales increase with wavelength (from hours at x-ray energies to days/weeks at optical wavelength and months/years at radio frequencies). During the last few years we have performed several simultaneous multi-frequency campaigns with high temporal resolution to study the nature of the fastest variations which probe the smallest regions in these AGN. Variability on timescales of hours to a few days were found to be common at optical and radio frequencies in a sample of bright radio-sources selected to have flat spectra in the 3-5 GHz regime.