Abstract
Active galactic nuclei with flat radio spectra exhibit significant variations of continuum flux density on time scales of days or less throughout the entire wavelength range. These rapid variations tightly constrain the diameters of the emitting regions and imply extremely high photon densities if the variations are intrinsic. At radio frequencies the flux densities of compact objects may flicker due to interstellar scintillation, and in all frequency bands microlensing by stars in intervening galaxies may introduce variations that are not intrinsic to the source. We review the characteristics of these variations in the various electromagnetic bands. Extrinsic mechanisms may affect the light curves of compact extragalactic sources, but close correlations between flares recorded in different bands strongly support the assumption that intraday variability is an intrinsic phenomenon. The apparent brightness temperatures in the radio regime exceed 1017 K and imply relativistic beaming with very high Doppler factors, coherent radiation mechanisms, or special geometric effects.
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