A&A 394, 17-31 (2002)
DOI: 10.1051/0004-6361:20020988

Correlated radio and optical variations in a sample of AGN

M. T. Hanski¹, L. O. Takalo¹ and E. Valtaoja<sup>1, 2

1</sup>  Tuorla Observatory (Tuorla Observatory is part of Väisälä Institute for Space Physics and Astronomy, University of Turku.) , 21500 Piikkiö, Finland
²  Department of Physics, 20014 University of Turku, Finland

(Received 17 April 2002 / Accepted 1 July 2002 )

Abstract
Radio light curves of 20 sources from the Metsähovi monitoring program are compared with optical light curves collected from the literature. The Discrete Correlation Function analysis is applied to the data sets. A new qualitative method to study correlations is introduced, where the radio light curves are replaced by model light curves consisting of exponential outbursts. The optical flux level is compared to the phase and flux level of the model flares. Seven sources show clear correlations using the DCF analysis with six more showing a possible correlation between optical and radio events with time lags from zero to several hundred days. For twelve sources at least one simultaneous optical and radio event is seen. For eleven sources, when comparing optical flux levels with the phase of the model radio outbursts, the optical flux levels were high at the peak of the model radio outbursts. For sixteen sources, when comparing the optical flux level with the radio model flare flux level, the optical flux level was high when radio flux level was high, as well.

Key words: galaxies: active -- BL Lacertae objects: general -- quasars: general

Offprint request: M. T. Hanski, mhanski@astro.utu.fi

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© ESO 2002

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