Cygnus X-3 has been observed with high sensitivity and spatial resolution at 6 cm wavelength on four consecutive days during a major flare in January 1991. The visibility amplitudes are low, in the range of 5-20% of the total flux density of the source, with minima down to the -detection limit of 10 mJy. The Green Bank Interferometer lightcurves at 13.3 and 3.6 cm show consecutive outbursts following a large flare starting on January 18, 1991 and, as can be also seen in the spectral index distribution, contain evidence that the apparent slow decay of the large flare is due to blending of continuous outbursts. Since the visibility amplitudes are low, a unique model cannot be derived from these observations. The best model representation of the data is of an almost continuous emission of length 60 mas along a position angle of .
We argue that the complex structure of the visibility amplitudes and the lightcurve is caused by a superposition of multiple flares forming a quasi-continous outflow of the source. Our data analysis shows that adequate modelling of the complex outflow characteristics of the radio emission of Cygnus X-3 during outburst requires continuous multi-wavelength monitoring, especially at short wavelengths. The VLBA with it's multi-wavelength capability should have enough flexibility to perform monitoring of the source immediately following a reported large flare. It is important to observe the peak of the flare with VLBI as early as possible, in order to follow (and be able to distinguish) multiple outbursts, which we have shown here to be likely to occurr. Short wavelength observations at 1.3 cm (or at 7 mm, depending on sensitivity, see Schalinski et al. in prep.) are less affected by scattering, and should allow to determine the ejection of new components close to the core, whereas observations at longer wavelengths, e.g. at 3.6 cm and 6 cm, are sensitive to the evolution of the complex jet. The complex outburst structure requires a sampling of at least two days, although continuous monitoring for at least 14 days would be preferred. Complementary interferometric studies of Cygnus X-3 during "quiescent" stages (c. the total flux density monitoring by Waltman et al. 1994) should reveal whether there is continuous outflow via jets.
© European Southern Observatory (ESO) 1998
Online publication: December 8, 1997