2 Periodicities in the stellar activity

V773 Tau was observed with the Effelsberg 100-m telescope over a frequency range spanning from 8 GHz (3.6 cm) to 40 GHz (7 mm). The monitoring covers a total time interval of 522 days during which time we collected 110 samples. No simultaneous measurements at different frequencies were performed. The observations have been performed as already described in Neidhöfer et al. (1993). The source was also observed with the VLA for one month centered around JD = 244411730 at a frequency of 14.96 GHz (2.0 cm). The sampling rate of two days resulted in 14 samples.

The total data set is presented in Fig. 1. Figure 2 shows the data folded with the orbital period of 51.075 days. Some flares are slightly displaced from the periastron passage (indicated with bars in Fig. 1 and related to phase = 0, 1, 2 in Fig. 2). However, a clustering is evident. The flare at 7mm clearly deviates from this general trend. The spectral analysis (Fig. 3) confirms a main period of 52 $\pm$ 5 days.

Figure 4 shows the flares around JD = 244411730 in detail. The peaks are 3.6 days and 3.4 days apart, respectively. Star spots on the surface of V773 Tau produce light variations with a period of 3.4 days as reported by Rydgren & Vrba (1983). Because the star spots are the foot points of the loops confining the radio emitting plasma, the optical and our radio variations can be explained by an active region quite stable in longitude, coming in and out of the line of sight as the star rotates. The energy distribution of the relativistic electrons, responsible for the emission, changes due to losses by synchrotron radiation and by Coulomb collisions over time. The fact that we still see a high density flux after a rotation (see Fig. 4) either implies that the flaring process lasts more than three days or that after one rotation a mechanism (perhaps inter-binary interaction) again activates the flaring process.