SIMBAD references

We have performed a correlation analysis of radio and optical variations in a large sample of Fermi blazars, of which there are 36 flat spectrum radio quasars and 34 BL Lacertae objects. The discrete cross-correlation function (DCF) method has been employed to investigate the correlation and the possible time lag. A clear correlation has been found in 55 sources (78.6% of the sample sources). It is confirmed that the variations in optical bands lead those in radio bands by several days to several hundred days. In four cases, optical flares are found to occur simultaneously with the radio flares. A jet-in-shock model has been used to explain these results. We estimate that the lag between the radio and optical bands may range from 3 days to 2.4 years. The DCF analysis results are consistent with expectations.