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Astron. Astrophys. 339, 41-51 (1998)

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4. Conclusions

This paper reports on the results of a radio, optical and photopolarimetric monitoring campaign on Mkn 421. The data quality, sampling, and total duration were sufficient to measure temporal correlations between optical and radio wave bands, with the optical emission that precedes the radio emission of almost 1-2 months. The most important result is the observation of a large increase of emission both at optical and radio wavelengths, with time-scales of almost 5 months, significatively longer than the lag measured between wave bands. The outburst was strictly correlated to a large increase of linear polarization. Moreover the optical polarization is higher at shorter wavelengths.

These results have important implications for theoretical models that want to explain optical and radio variability of BL Lac objects. The most natural explanation seems to be the multi-component model or its natural extension, the inhomogeneous jet model. The observation that the optical emission leads the radio one by less than the variability time-scale argues for a flattened geometry, such as might be expected at a shock front in the jet. We discussed how the variability can be explained if the spectra observed during flares are superpositions of spectra arising from spatially separated sources (shocks or reconnection sites) in the jet. In this contest the large optical-radio outbursts are generated by relatively long-lived sources placed in the outer parts of the jet, while rapid keV and TeV flares are produced closer to the central engine.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998