## 7. DiscussionThe analysis of the total flux density light curves at mm and cm
wavelengths, together with multi-epoch, multi-wavelength VLBI data,
allows us to suggest that the 4-year cycle of the total flux density
variations is connected with the regular ejection of components with
apparent superluminal motion. The VLBI data reveal the existence of a
superluminal component ( According to Kellermann & Pauliny-Toth (1969) and Marscher (1987) the magnetic field [Gauss] can be estimated from the turnover frequency [GHz], corresponding flux density [Jy], and angular diameter of the emitting component (approximated as a uniform sphere) at the turnover frequency [mas]: The spherical angular diameter of a component can be found from the elliptical gaussian FWHM of the VLBI model fit by the relation (Marscher 1987). From =8.1 we determine the minimum possible Lorentz factor 8.2; the viewing angle for can be estimated by the formula , and the Doppler boosting factor in this case is close to the value of the Lorentz factor 8.2, but it can be much smaller for larger angles (up to and much larger for smaller angles. For the core outburst (see Fig. 5), the turnover frequency 37 GHz and the corresponding flux density =1.5 Jy. The mean elliptical gaussian size of the core from our 43 GHz VLBA data =(0.0850.035) mas (Table 3) and the corresponding spherical diameter mas. We then obtain for the magnetic field of the core observed at 40 GHz a value G. The corresponding time scale in the observer's frame for synchrotron losses of electrons at 37 GHz is yr. The shortest timescale of variability observed (from the data shown in Fig. 1) is 0.6 yr (calculated from the formula ; Burbidge et al. 1974). Therefore, either the observed timescale corresponds to one of the following: (a) changes in the acceleration rate of relativistic electrons, (b) changes in strength of the magnetic field, or (c) value of the light-travel time across the emitting region, or the magnetic field is lower than the value derived above. This latter possibility would occur if there is substructure within the core: the core is only slightly resolved by our observations and could contain two or more smaller subcomponents. If this is the case, the magnetic field could be as low as G, which is estimated from the condition . Fig. 14 shows that outburst 2 (and probably II) coincides with
the moment when jet component The appearance of a 1.3-yr periodicity, which is especially prominent in the high frequency light curves (see Fig. 7 and Table 1) is interesting. This could be caused, for example, by components passing through periodic compressions and rarefactions in the jet, generated by pressure imbalance with the external medium (e.g., Gómez et al. 1997). © European Southern Observatory (ESO) 2000 Online publication: June 8, 2000 |