Optical monitoring of radio sources

Abstract

This paper presents new results from the Herstmonceux Optical Monitoring Programme for a selection of quasars, BL Lac objects and different types of active galaxies. The light curves given cover the period ∼ 1966–1980. Comparison with variations at other wavelengths indicates a similar type of behaviour in the optical and X-ray regions and a poor connection between optical and radio variations. An analysis of a large body of data from the literature shows that the Hubble diagrams of QSOs with flat and steep radio spectra are indistinguishable (in contrast to earlier work). This is supported by the similar degrees of evolution recently determined for radio source populations with flat and steep spectra. It is also shown that neither the simple shot noise nor the periodic model provides an adequate description of optical variability. Relativistic beaming does not make a significant contribution to the quiescent optical luminosity nor to the optical outbursts.