6 Conclusions

The main results of this paper can be summarized as follows:

1.

We have reported VLA radio observations that clearly reveal the development of bipolar, two-sided radio jets in the microquasar Cygnus X-3 in the weeks after a series of strong flaring episodes. Our VLA maps confirm the two-sided appearance of the radio jets when imaged at large distances from the binary system (>0.01 pc).

2.

The proper motions of both the jet and the counterjet have been estimated based on the most likely ejection date. For the northern jet component, our estimate is further consistent with a direct proper motion measurement between two consecutive epochs separated by two weeks. All this can be used to constrain the ejection parameters of the system. In particular, we obtain a value of $\beta \cos{\theta} = 0.14 \pm 0.03$ that is in excellent agreement with an independent estimate based on the Doppler boosting effect. Assuming a 10 kpc distance, we derive an ejection velocity of $(0.48 \pm 0.04)c$ with an angle of $73\pm 4^{\circ}$ with the line of sight. No apparent superluminal velocities are observed.

3.

Considering the new high values of the wind mass loss in Cygnus X-3 based on ISO data, as well as the likely disc-like structure of the wind, we tentatively suggest that the one-sided radio jet seen by Mioduszewski et al. (2001) is due to absorption effects in the vicinity of the binary system and not to a jet angle very close to the line of sight. If this is correct, the amount of Doppler boosting is better measured at the angular resolution of the VLA where absorption is negligible. Other interpretations discussed in the text are nevertheless not completely excluded.

Acknowledgements

We acknowledge partial support by DGICYT (PB97-0903), AYA2001-3092 and Junta de Andalucía (Spain). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Green Bank Interferometer is a facility of the National Science Foundation operated by the NRAO in support of NASA High Energy Astrophysics programs. The authors also thank L. F. Rodríguez (UNAM, México) and an anonymous referee for helping to improve this paper, and G. G. Pooley (MRAO, UK) for making possible the on-line consultation of the Ryle Telescope monitorings.