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

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2. Observations and results

The observations were carried out by using the VLA 1 in different runs in the period 1991 October to 1992 April, covering different orbital phases. We observed at five frequencies, namely 1.49 (L-Band), 5.0 (C-Band), 8.4 (X-Band), 14.9 (U-Band) and 22.0 (K-Band) GHz, using two independent 50 MHz bands.

The observations were performed with the entire array used sequentially at each frequency. This ensures the necessary sensitivity for detection of weak sources and is suitable for sources whose variability is on timescales of an hour or longer. At each frequency a typical observing cycle consisted of 20-min integration time, preceded and followed by a 2-min observation of the phase calibrator (0224+671).

When possible, this sequence was repeated in order to improve the signal to noise ratio. The flux density scale was determined by daily observations of 3C286 and 3C48. The data were calibrated and mapped by using the standard procedures of the Astronomical Image Processing System (A.I.P.S.). We first mapped the data to find out if other sources were present in the field of view of VLA. Once problems due to possible confusion of sources were excluded, each scan was analyzed with the AIPS program DFTPL. This procedure performs the direct Fourier transform of the visibility function in a limited portion of the UV plane.

Selecting the position of the radio source, as derived from the map, it was possible to analyze the temporal behaviour of our target, with high temporal resolution and thus to locate rapid variations of the radio flux density. The results of such an analysis led to the conclusion that, even if the radio flux of RZ Cas is variable, it does not change significantly on a timescale of 20 minutes or less.

The five point spectrum was then obtained by deriving the flux density at each frequency from the cleaned map integrated over 20 min and we assume the rms of the map to be the uncertainty in the flux density. When, on the same day, two observations at each frequency were made, we derived two different values of the flux density by mapping the data relative to each scan.

Table 2 summarizes the results of these observations as follows: column 1 the date of observations, columns 2 and 3 the time of observations (UT), columns 4, 5, 6, 7 and 8 the measured radio flux density or an upper limit, with the associated rms error, at the various frequencies. Multiple values of flux density represent the results obtained by using all the data (mean value) or those for each separate scan. The measured flux densities show a large spread, but in a range consistent with previous VLA observations (Drake et al. 1986; Umana et al., 1991; Umana et al., 1993).


Table 2.

To exclude any spurious variations in the data of different epochs due to flux calibration, we compared the time behaviour of the radio flux of 0224+671 with that of RZ Cas at different frequencies. As an example, we plot in Fig. 1 the flux of the calibrator at 5 GHz, as percentage of the averaged value, together with radio flux of RZ Cas. It is evident that the calibrator flux did not vary within 10% while RZ Cas showed variations up to 200%. The same trend is observed in the other bands. As already pointed out, the radio emission of RZ Cas is variable at all the observed frequencies.

[FIGURE] Fig. 1. Percentage variations of the 5 GHz flux density as a function of time (Julian Day) for RZ Cas and its phase calibrator. Each point represents the variations, as percentage with respect to the mean value of the radio flux density. Empty squares refer to 0224+671, while filled dots refer to RZ Cas

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999