Correlation between the five-day integrated γ-ray light curve and the optical (top) and radio (bottom) light curves of 3C 345. The correlation is obtained by calculating the discrete correlation function (DCF) with ten-day binning. A positive time-lag corresponds to the γ-ray variations preceding the variations in the other two bands. The significance of the correlation is illustrated by 3σ (99.7%) significance contours calculated from simulated, uncorrelated data with different slopes, β, of the power spectral density (PSD). The contours are drawn for the PSD slopes β  =  1.0, 1.5, 2.0, and 2.5 (indicated by progression of colors, from light gray to black). For the simulated data, values of the correlation coefficient exceeding  ± 1 are an inherent feature of the method, resulting from differences in overlap of the time series at different time lags (cf. Edelson & Krolik 1988; Timmer & Koenig 1995, and the description of the applied method in Sect. 3.4).