Cygnus X-3 was originally discovered in X-rays by Giacconi et al. (1967). This X-ray binary
has been the subject of intensive study during the last decades,
specially after the discovery of its giant radio outbursts in 1972
(Gregory et al. 1972). Today, it is widely accepted
as one of the few known microquasar sources in our Galaxy
(Mirabel & Rodríguez 1999). The black hole or neutron
star nature of the compact companion in the binary system
is still a matter of debate, but a consensus has been reached about the normal companion
being a WN Wolf-Rayet star (van Kerkwijk et al. 1996).
With this classification, the system belongs to the group of high mass X-ray binaries.
The modulation observed in the X-ray and infrared emission every 4.8 h (e.g. Mason et al. 1986)
is interpreted as the orbital period of the system. Thus, the orbit of the compact object
around the WN Wolf-Rayet star has to be very tight with semimajor axis of only a few solar radii.
Cygnus X-3 has been also proposed to be an emitter of ultra high energy 
-rays by
several groups (see e.g. Murthy & Wolfendale 1993). The recent discovery of another
high mass X-ray binary, LS5039, with a likely -ray counterpart (Paredes et al. 2000)
points to microquasars being possibly connected with some galactic -ray sources.
Focusing on the Cygnus X-3 flaring radio emission, several authors have provided strong observational
evidence indicating that it originates in expanding collimated jet-like structures
(see e.g. Martí et al. 2000 and all the references therein).
The highest resolution maps of the ejecta have
been provided by Mioduszewski et al. (2001)
who observed Cygnus X-3 with the Very Long Baseline Array (VLBA) soon after a giant
outburst event in 1997. Further support for the jet scenario
comes from the agreement between the radio light curves and the
predictions from theoretical models of synchrotron emitting
radio jets (see e.g. Hjellming & Johnston 1988; Martí et al. 1992).
In spite of this progress, some questions concerning the physical properties of the
ejecta remain not yet fully understood:
are the jets strongly beamed towards us? What is the true velocity of the flow?
Where is the core in the high resolution maps?
The present paper reports new multi-epoch radio images of Cygnus X-3 and its radio jets.
Our observations were obtained
in ToO mode, with the
triggering event being the recent giant radio outburst in September 2000.
Most ToO radio observations of Cygnus X-3 have
consisted of mapping the source using Very Long Baseline Interferometry (VLBI) arrays
with milli-arcsecond (mas) angular resolution. These VLBI runs are normally carried out
as soon as possible after the outburst onset.
Contrary to this standard approach, we
waited and observed several weeks after the onset, with the expectation of studying and imaging
the ejecta far away from the central core.
The suitable instrument for this project was
the VLA of the National Radio Astronomy Observatory (NRAO).
Our previous VLA maps of the 1997 outburst, with the central variable core carefully
subtracted, indicated that the Cygnus X-3 ejecta is indeed detectable up to one arcsec
away from the ejection center (Martí et al. 2000).
With these ideas in mind, the main goal of our ToO proposal
was to confirm the reality of the extended bipolar radio jets
and to obtain a convincing record
of their proper motion.
The resulting maps have an angular resolution of 0
3, equivalent to about 0.01 pc at a
distance of 10 kpc. Since phase calibration is straightforward with the VLA at cm wavelengths,
the maps have no ambiguity in the location of the core, as it happens in some VLBI images.
In the following sections, we present the confirmation that
Cygnus X-3 is able to develop transient radio jets at arcsecond scales
and that they are moving with relativistic speeds. Moreover,
the new VLA results open the challenging question of how to relate, consistently,
the elongated radio structures seen at very different angular scales.
Copyright ESO 2001