5 Discussion

According to Valtaoja et al. (1992), optical and radio outbursts can happen either simultaneously or with time lags growing towards lower frequencies. The difficulty is that in any source there can be outbursts both with and without time lags. With this kind of situation the DCF analysis is not a perfect tool for finding correlations. Also, the optical outburst are fast, and because of the gaps in the light curves, they are easily missed. The radio outbursts are slower, but they are often overlapping: a new radio flare begins before the previous outburst has decayed. In such cases it is difficult to determine which optical outburst a certain radio outburst is associated with.

Another problem is that there are probably other mechanisms triggering optical outbursts than just shocks in a jet. In the optical regime there are at least two emission components: synchrotron and thermal. The synchrotron component probably is of the same origin as the radio emission, which is also synchrotron emission. The thermal optical emission, on the other hand, probably has no radio counterpart. This means that in one source optical flares can occur both with and without radio counterparts. Such is the case in the best studied source OJ 287 (Valtaoja et al. 2000).

Also, it seems that "too much'' data makes correlation analysis difficult. Throughout this analysis the optical data was binned in one day bins. Normally this binning is good, because there are not that many data points. However, for some sources in this study, namely OJ 287, BL Lac, 3C 66A and AO 0235, it might be reasonable to use larger bins, say a week for optical light curves. The best bin size would probably be approximately that used for the radio data, too.