We have not detected short-term variability in Cen A. The detection of
percentage variations of extremely bright sources can be limited by the dynamic
range of the receiver. However, in the case of the present observations we have
a range of 
which allows us a proper determination of
fluctuations on the percent level for sources with flux densities even
beyond 
. Consequently, we conclude that significant flux
density variations in Cen A seem to be confined to timescales larger than
1 month at 1.4 GHz. At larger frequencies, however, there are reports of strong
rapid changes in flux density (for instance, Kellerman 1974
has observed 50% fluctuations over one day at 88 GHz). Our data (see
Fig. 3 (click here)) show variability
with amplitudes 
over timescales of 
. The
intensity parameter 
is about 1.23 Jy/day, implying an upper
limit for the
linear size of the emitting region of 
. The corresponding
brightness temperature for the synchrotron plasma that produces this emission
(
) is 
, a value quite
smaller than the inverse Compton limit. There is no need, consequently, for
positing relativistic bulk motions in the source in order to reduce the
derived brightness temperature in the observer's frame. This is in
accordance with recent VLBI observations of Cen A which show subluminal
velocities of 
in the inner jet of the object (Preston et
al. 1996). The observed variability might be produced when a shock
strikes a small feature (e.g. a density inhomogeneity) in the pc-scale jet
(Romero et al. 1995c). This interpretation is supported by
the knotty structure of the inner radio jet observed, for
instance, by Burns et al. (1983). Besides, there seems to be
unlikely that the variations could be originated in the core alone due to
its relatively low contribution to the total flux density at 1.4 GHz.
With the exception of small fluctuations between JD 2449866 and JD 2449868, and JD 2450016 and JD 2450030, we have observed no variability in the gravitational lensed system PKS 1830-211. Gravitational microlensing might cause variability in this source: compact objects like stars or brown dwarfs belonging to the lens-galaxy can magnify the emission from the nucleus or a superluminal component in the jet of the background AGN producing rapid and symmetric changes in the flux density (e.g. Nottale 1986; Gopal-Krishna & Subrahmanian 1991). Variations over timescales of days require extremely high velocities of the lens with respect to the observer or, more reasonably, a superluminal lensed source. Our fail in clearly detecting these variations suggests that superluminal components with a significant part of the total flux density were not present in the source during the observational period. Future variability observations with better sensitivity of PKS 1830-211 could provide a valuable tool for investigating the nature and structure of the background source and the interposed galaxy, assuming the corresponding redshifts can be obtained (see Romero et al. 1995b for a treatment of this kind).
The QSO 1610-771 presented the larger variability amplitudes of the
sample. At intraday timescales there is no variability over the
observational errors, but at timescales of months variability amplitudes of
were observed. Superposed with this variability there are rapid
fluctuations of a few days. The fastest changes in flux density have a
peak-to-peak amplitude of 
in 4 days. The intensity
parameter 
is of 
for these variations.
This kind of events implies large brightness temperatures well beyond the
inverse Compton limit (
, for 
and 
) if they are interpreted as
intrinsic to the source. Shocked jet models with favorable geometries (e.g.
Qian et al. 1991; Romero et al. 1995c) require
bulk Lorentz factors as high as 15 to account for these observations.
Conversely, the temperatures derived from the intermonth variability data
are easily reconcilable with the standard shock-in-jet model of blazars
(e.g. Marscher 1992). One possibility to be considered is that
the fast variations are produced by refractive interstellar scintillation
(Rickett 1986) whilst the variation over larger timescales
could have an intrinsic origin. This hypothesis is supported by the fact
that the interday structure function is linear in T for T small, in
agreement with the theoretical predictions for the variability produced by
an extended scattering medium (Blandford et al. 1986). The
observed fluctuation index implies, in this interpretation, that a
significant part of the flux density is within a 
core.
Beyond the origin of the short-term variability of PKS 1610-771, it is clear that this object should receive more attention in the future.
Acknowledgements
We thank E. Hurrell for assistance during the observations, and A. Bava and J. Sánz for technical advise. This work has been partially supported by CONICET and UNLP.