SIMBAD references

We present the results of radio, optical and photopolarimetric observations of Mkn 421 from October 1994 to June 1997. During our monitoring the object showed many flares and major outbursts over a large span of temporal scales. A large optical outburst occurred in the winter 1996-97, followed by a radio outburst with a delay of only 30-60 days. In the same period Mkn 421 increased its polarization level toward values never observed before. From our data a clear correlation between the colour index and the brightness level appears, which may be due to the thermal contribution of the host galaxy. The frequency dependence of the optical polarization cannot be due to the host galaxy dilution and, therefore, it is related to the physics of the blazar synchrotron emission and to the geometry of the source. The most natural explanation seems to be the inhomogeneous jet model, and the outburst behaviour is such as might be expected at a shock front in the jet. We discuss how the optical 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.