Since 1992 several observational campaigns searching for rapid
variability of southern extragalactic radio sources have been carried out
at the Instituto Argentino de Radioastronomıa (IAR) (Romero et al.
1994, 1995a; Romero & Combi 1995). These campaigns
were conceived with the aim of gathering a set as complete as possible of
radio variability data of southern active galactic nuclei (AGNs) on
timescales ranging from hours to months at relatively low radio frequencies
(
). Together with the abundant data concerning the behaviour of northern
sources (e.g. Quirrenbach et al. 1992) they can provide
elements for an all-sky statistics of rapid radio variability of AGNs. In
this paper we present results of new observations of the sources PKS
1610-771, PKS 1830-211, and the central region of Cen A, with different
temporal resolutions.
PKS 1610-771 is a strong (
),
flat-spectrum (
, 
) source with a steep optical spectrum (Hunstead &
Murdoch 1980). It has been classified as a QSO by
Véron-Cetty &
Véron (1996). The object, with a redshift z = 1.710, has been
observed at high angular resolution by the Southern Hemisphere VLBI
Experiment (Preston et al. 1989). These observations indicate
a 3.8 Jy elliptical Gaussian core elongated 10 mas along a position angle of
, and a 1.4 Jy circular Gaussian halo of 25 mas in radius at 2.3
GHz. The source, consequently, seems to be compact with no visible structure
over the 10 mas. As far as we know there are no reports about radio
variability for this object until now.
PKS 1830-211 is a strong, flat-spectrum source with a double structure at
scales of 1 arcsecond. VLBI observations have shown the presence of an
Einstein ring and two almost identical bright spots, which have been
interpreted as images of a background lensed object (namely a QSO). Owing
to its gravitationally lensed nature this system has call much attention
in recent years (see, for instance, Jauncey et al. 1991; Nair
et al. 1993; van Ommen et al. 1995). The total flux
density at 1.4 GHz is 
(Pramesh Rao & Subrahmanyan 1988). Flux density
variability has been found at 2.3 and 8.4 GHz by Lovell et al.
(1996). At the latter frequency the source evolved from a flux
density value of 
in late 1990 to a peak of 
in early 1992, and then decreasing up to 
. Variability
information about this object is important in order to constrain the
proposed lens models (e.g. van Ommen et al. 1995).
The third source of our sample, Cen A, is the nearest active galaxy: its
distance is 
(Hui et al. 1993). It
is a complex and extremely powerful source in the southern sky, covering an
area of about 
. The large scale radio structure
has been investigated by Cooper et al. (1965), Junkes et al.
(1993) and Combi & Romero (1996), among others. The
major radio components of the source are the Northern and Southern Giant
Lobes, the Northern Middle Lobe, the Double Inner Lobes, the jet (also
visible at X-rays), and the mas-core (see composed maps in Meier et al.
1989). A weak counterjet has been recently discovered with a global
(SHEVE + VLBA) array (Jones et al. 1996). The inner radio
structure was studied in detail by Burns et al. (1983) with the
VLA. Very rapid variability of the central regions of Cen A at
mm-wavelengths has been reported by Kellerman (1974) and
Kaufmann & Raffaelli (1979), and variability over larger
timescales was detected by Abraham et al. (1982),
Botti & Abraham
(1993) and Abraham (1996), among others. Several authors
have suggested that Cen A may
harbour a misdirected BL Lac object (e.g. Bailey et al. 1986;
Morganti et al. 1992). In such a case variability at
cm-wavelengths might be produced in the inner region of the source over
relatively short timescales (may be months).
In the next section we shall briefly describe our observations of the selected objects. The following sections are devoted to the results and their discussion.