1932

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.

Loading

Article metrics loading...

/content/journals/10.1146/annurev.aa.33.090195.001115
1995-09-01
2024-06-19
Loading full text...

Full text loading...

/content/journals/10.1146/annurev.aa.33.090195.001115
Loading
  • Article Type: Review Article
This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error