SIMULTANEOUS MULTIFREQUENCY OBSERVATIONS OF BLAZARS

Among active galactic nuclei, blazars have the most highly and. rapidl y variable spectral continua at all the wavelengths. Their repeated ob servation at different energies allows therefore to investigate the ge ometry and the physics of the emitting regions, provided the multifreq uency data are monitored simultaneously and with complete and frequent sampling, to allow a reliable correlation analysis of the light curve s at different wavelengths. The coordinated efforts of several interna tional astronomical groups toward multifrequency monitoring implementa tion allowed to obtain unprecedentedly high quality data and to achiev e substantial improvement in the theoretical understanding of the blaz ars PKS 2155-304 and 3C 279.