2. Observations and data reduction
We have monitored at 1.4, 5, 8.4 and 15 GHz the B9p Si star CU Vir (= HD 124224) over three separate days using the VLA 1. The observations have been carried out on June 1, 6 and 11, 1998 from 23 to 07 UT with all the available telescopes at the beginning of the A to B reconfiguration. For each frequency, we used the standard observing mode with two independent 50 MHz bands in Right and Left Circular Polarizations (RCP and LCP), adopting a 10 sec integration time. At 1.4 GHz the two bands are separated by 80 MHz, being centered at 1385 and 1465 MHz.
A typical observing cycle consisted of 10-min on source preceded and followed by 2-min on the phase calibrator 1354-021, which is only far from CU Vir. The four frequencies have been observed alternately with all the telescopes, so that at each frequency CU Vir has been observed for about over a total time of . The sequence of the frequencies during the three observing runs have been organized to get the best possible sampling of the rotational phases, avoiding redundancies. To get a reliable flux scale, the amplitude calibrator 3C286 was observed at the beginning and the end of each run ( and of elevation respectively).
Data were calibrated and mapped by using the standard procedures of the Astronomical Image Processing System (A.I.P.S.). CU Vir was found unresolved on VLA baselines at all the observed frequencies. Its position was determined by using the task JMFIT, and found to be coincident with the position given in the Hipparcos Catalogue (Perryman et al. 1997): and . The temporal variation of the Stokes I and V parameters was determined with the task DFTPL. This task performs the direct Fourier transform of the visibilities as a function of time for an arbitrary position in the map. The results of this task can be affected by the sidelobes of any other strong source in the field. However, since the only source close to our target is very weak, having a flux density of only 3 mJy at 1.4 GHz, the possible confusion is negligible. Heliocentric correction was then applied to the times of observation. Fig. 1 shows the radio light curve at 1.4 GHz during the three days of observation.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100