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Astron. Astrophys. 342, 709-716 (1999)

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3. The behaviour of the radio variability

We performed the radio observations of RZ Cas over a wide time interval of about 7 months, starting on October 1991. Due to our irregular sampling, it is not easy to assess a typical time scale for the variability.

In an eclipsing binary the most obvious time scale for variability is the orbital period. We thus checked if the observed variability may be related to a geometrical effect, as the sky projection of the radio source changes as the system rotates. However, no correlation with the orbital phase is evident from our data, as one would expect in case of rotational modulation of the radio flux, assuming that the rotational period of the secondary star is synchronized to the orbital period.

Instead the radio emission seems to be characterized (see Table 2) by periods of low flux density level (0.7-1 mJy) and a high (2-3 mJy), highly variable level, that we will refer to as an active period. On previous observations RZ Cas was found at intermediate, 1.35 mJy (June, 1985) and 1.25 mJy (Feb.1989) as well as high emission levels, i. e., 3.45 mJy (Nov. 1984) (Drake et al., 1986; Umana et al., 1991).

From the flux densities, measured at each frequency and shown in fig 2, it seems that in the active period (panel b), i.e. on Feb. 1992, a flare like event has occurred around JD 2448666, i.e. in the second half of the observing period, which probably peaks between 15 and 16 February. It is interesting to note that the flux density appears to rise first at the higher frequency (K-Band) and only subsequently in the other ones. The flux enhancement appears very marginal in the L band.

[FIGURE] Fig. 2a and b. Flux densities observed at each frequency as functions of time (Julian Day). In panel a all the observations are reported, while in panel b the observations relative to the active period are plotted in an enlarged scale. The rms, associated to each measurement, lies well inside each square symbol. 

Whether the high state is the result of recurrent flares, similar to those characterizing the RS CVn binaries, for example HR 1099 (Feldman et al. 1978, Umana et al. 1995) and UX Ari (Trigilio et al. 1998), it is not possible to state, because of the lack of continuous observations.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999