The search for correlated events between different frequency regimes in active galactic nuclei (AGN) continues. This is due to the increasing amount of data available for more and more sources, thanks to long term monitoring programs. It is clear that there is a connection between optical and radio variability at least in some sources, e.g. 3C 279, 3C 345 (Tornikoski et al. 1994b) and AO 0235+164 (Clements et al. 1995; Raiteri et al. 2001). The smooth UV-to-radio continuum spectra of many AGN indicates a possible connection between these regimes. Also along with radio jets optical jets have been seen in some sources, e.g. 3C 273 (Guthrie & Napier 1975), M 87 (Lelievre et al. 1984), 3C 371 (Nilsson et al. 1997) and 3C 380 (O'Dea et al. 1999).
It is generally accepted that radio outbursts in AGN are triggered by growing shocks in a jet that shoots out from the galaxy center (Marscher & Gear 1985; Valtaoja et al. 1992; Türler et al. 2000). The generalized shock model (Valtaoja et al. 1992; Lainela 1994) explains how a shock grows and decays, and how the outburst looks at different frequencies. The model predicts what kind of time lags are expected between outbursts at different observing frequencies.
So far, light curves are the only way to study the origins of shocks in a jet. Even the innermost parts of jets cannot be resolved with the Very Long Baseline Interferometry (VLBI). The shock component can only be seen in VLBI maps after the most interesting growing phase of the shock is over long ago. If a radio outburst is modelled with an exponential flare (see Sect. 3.2), the observed VLBI component becomes, in general, visible when the shock is already decaying and 2/3 of the model outburst has already taken place (Savolainen et al. 2002).
Correlations between different frequency regimes in AGN have been searched for since the beginning of the 1970's. Individual objects have been studied in many papers, e.g. 3C 120 (Usher 1972), BL Lac (Andrew et al. 1974; Tornikoski et al. 1994a), PKS 0420-01 (Dent et al. 1979), 3C 446 (Bregman et al. 1988), OJ 287 (Kikuchi et al. 1973; Kinman et al. 1974; Usher 1979; Valtaoja et al. 1987) and AO 0235+164 (Balonek & Dent 1980; Roy et al. 2000; Raiteri et al. 2001).
The first studies containing several sources were carried out by Usher (1975) and Pomphrey et al. (1976). Usher (1975) studied the correlation between radio spectral index and optical variability. Pomphrey et al. (1976) studied optical and radio light curves and found a strong correlation between optical and radio only in one object, OJ 287. A very ambitious search for correlations in 45 AGN's was done by Balonek (1982; hereafter B82). Of the sources studied, 11 exhibited a likely correlation between optical and radio with time lags up to several hundred days. Correlations were not found in 15 sources. Other extensive studies were done by Tornikoski et al. (1994b; hereafter T94) and Clements et al. (1995; hereafter C95), who both studied a large sample of AGN.
In T94 a clear correlation between optical and radio was found in 10 sources of the 22 studied. In C95 nine of the 18 sources showed radio-optical correlations. In both these studies and in this paper, the discrete correlation function (DCF) is used for correlation analysis. It is, however, insufficient to calculate the DCF values alone, because sometimes the DCF gives results that are not real features. Other methods are needed to confirm the results from the DCF analysis. In T94, C95, and this paper, the DCF analysis is complemented with visual inspection. We introduce also a new qualitative method for studying correlations.
We compare the optical light curves with the 37 and 22 GHz light curves for several quasars and BL Lac objects, first by using the DCF, and also by modelling the radio light curve with a set of exponential flares, and comparing optical flux level with the phase and the flux level of the concurrent radio flares.
| Source | Other names | Redshift | Type of Object | References | Previous correlation studies |
| 0109+224 | S2 0109+22 | ? | BLO | 5, 11, 13, 15 | - |
| 0219+428 | 3C 66A | 0.444 | BLO | 5, 7, 13, 14, 15 | B82 |
| 0235+164 | AO 0235+16 | 0.94 | BLO | 1, 9, 12, 15 | BD80, B82, C95, W00, R01 |
| 0420-014 | PKS 0420-01 | 0.915 | HPQ | 1, 8, 12 | P76, D79, B82 |
| 0422+004 | PKS 0422+00 | 0.31 | BLO | 4, 5, 6, 8, 13, 14, 15 | - |
| 0735+178 | PKS 0735+17 | >0.424 | BLO | 1, 5, 12, 13, 14, 15 | U75,P76, B82, HB92, T94, C95 |
| 0736+017 | PKS 0736+01 | 0.191 | HPQ | 5 | U75, P76, B82, T94 |
| 0754+100 | OI 090.4 | 0.66 | BLO | 3, 4, 5, 13, 14, 15 | - |
| 0851+202 | OJ 287 | 0.306 | BLO | 1, 5, 7, 12, 13, 14, 15 | U75, P76, U79, B82, V87, HB92, T94, C95 |
| 1156+295 | 4C +29.45 | 0.729 | HPQ | 4, 5, 8, 11, 12, 13 | T94, C95 |
| 1219+285 | W Comae | 0.102 | BLO | 5, 10, 12, 14, 15 | U75, B82, T94 |
| ON 231 | |||||
| 1226+023 | 3C 273 | 0.1583 | LPQ | 3, 5, 8, 11 | U75, P76,B82, T94, C95 |
| 1253-055 | 3C 279 | 0.5362 | HPQ | 5, 11 | B82, T94 |
| 1633+382 | 4C +38.41 | 1.814 | LPQ | 5, 8, 11 | - |
| 1641+399 | 3C 345 | 0.5928 | HPQ | 1, 5, 8, 12 | U75, B82, HB92, T94 |
| 1749+096 | 4C +09.57 | 0.322 | BLO | 4, 15 | B82, T94, C95 |
| 1807+698 | 3C 371 | 0.051 | BLO | 5 | U75, P76, B82, T94 |
| 2200+420 | BL Lac | 0.0686 | BLO | 2, 5, 12, 13, 15 | U75, P76, B82, B88, HB92, T94, C95 |
| 2223-052 | 3C 446 | 1.404 | HPQ | 1, 12 | U75, P76, B82, HB92, T94, C95 |
| 2251+158 | 3C 454.3 | 0.859 | HPQ | 1, 5, 8, 12 | U75 |
Copyright ESO 2002