The observational results presented in the previous sections are the first detailed description of the optical activity of ON 231 in a very bright state and are useful in understanding the physical processes responsible of such high variability.
We searched for possible recurrent features in the light curve of Fig. 2 for time intervals longer than ten days by computing the first order structure function averaging the data over time intervals of 5 days. The result is shown in Fig. 6: the structure function is quite smooth with a mean slope of 0.44 and only a swallow minimum around 90 days. A direct inspection of the light curve suggests that the main phenomena can occur at least on three possible time scales. The longest of the order several months (or years) corresponding to the long term trend, an intermediate scale of about 3 106 s (month) corresponding to the duration of the ouburst and a third one of about 105 s (1-2 days) associated with the flaring activity. The longest time scale could be produced by structural modifications of the source, the second one could be related to the acceleration and radiative cooling of relativistic electrons in the synchrotron source, and the third one to the development of short living instabilites or fast shocks inside the emitting region. A more complete analysis, however, is necessary to achieve a better description of the variability pattern useful to unravel the relations among the main parameters of a physical model, but it is not the aim of this paper.
The data on frequency dependence polarisation in different luminosity states are also very useful to understand the physics of the source. The polarisation fraction had a large increase at the same time of the onset of the ouburst and remained high without great changes of the polarisation angle, indicating that the field orientation was rather stable. Ballard et al. (1990) showed that a frequency dependent polarisation can be explained by the combination of two components: an unpolarised one with power law spectrum superposed to a highly polarised one having a flatter spectrum with a cut-off frequency, where the resulting polarisation is maximum. A luminosity decrease of the latter component cannot imply a lower polarisation degree if also the cut-off frequency decreases.
We stress, finally, some similarities between this outburst of ON 231 and that of BL Lacertae in summer 1997. In both cases a rapid flaring activity, with typical rates of about 0.2 mag/hour (Nesci et al. 1998, Speziali & Natali 1998), superposed to an increase of the mean luminosity was observed (Tosti et al. 1999c), The overall durations of both outbursts were on the month time scale. During the bursts the spectral distributions were harder than in quiescent phases and also significant enhancements of the X-ray luminosities were observed (Makino et al. 1999, Tagliaferri et al. 1998). All these analogies suggest that these intense outbursts of both sources are likely produced by the same physical processes.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999