Correlation of Ultraviolet and Radio Flux in Three Active Galactic Nuclei

Abstract

We report the results of a search for correlations between the time- variable ultraviolet (UV) and the radio flux in three AGNs, viz., 3C 273, 3C 120, and PKS 2155-304. Data from 1984 to present were used. UV data from the short-wave IUE camera were downloaded from the IUE Regional Data Analysis Facility and the National Space Science Data Center at NASA/GSFC. Radio flux at 4.8,8.0, and 14.5 GHz were obtained from the University of Michigan Radio Astronomy Observatory by Aller et al. The UV flux at 1550 A and UV spectral index were obtained by fitting power law to the short-wave continuum of each IUE spectrum. Cross-correlation and discrete-correlation functions were computed for the UV and radio fluxes over a common time interval of more than 5 yr. Cross-correlation analysis was also carried out between spectral indices in the UV and radio bands for the three objects. Significant correlations were found at one or more time lags for each of the three objects. The possibility that cross correlation analysis can yield delusive results is discussed.