PKS 1921-293 is one of the strongest (up to 
20 Jy), flat-spectrum radio sources in the sky, but was not identified by EGRET in greater than 100 MeV gamma-rays. It is a highly polarised and optically violently
variable quasar (OV-236) at a red shift of z=0.352 [Pica et al. 1988].
Fichtel et al. [1994] place an upper limit to greater than 100 MeV emission
from PKS 1921-293 of 0.6 
10 
cm 
s 
. It
was not considered by von Montigny et al. [1995b] in their
investigation of radio sources not identified by EGRET whereas they did consider the
similar sources PKS 0438-436 and 0637-752. As one of the most
extreme blazars, and one of the brightest core dominated radio sources PKS 1921-293 clearly should be considered in any
comparison between gamma-ray loud and quiet AGN.
PKS 1921-293 was observed by Preston et al. [1989] with the
first multi-baseline VLBI observations at a frequency of 2.3 GHz.
They found the radio source to consist of a 5 mas elongated component at a
position angle of approximately 25 
and with a flux density of
6.6 Jy. The new VLBI observations at 4.8 GHz are represented by
the image in Figure 4.12. The image shows that the compact radio source is
dominated by an unresolved core component and a jet-like structure at a
position angle of approximately 25 , in agreement
with the results of Preston et al. [1989]. PKS 1921-293 was
completely unresolved at the VLA at 1.4 and 5 GHz as observed by de
Pater, Schloerb and Johnston [1985] and Perley [1982]. Thus, no comparison between the mas-scale and arcsecond-scale structure position angles can be made.
Figure: Low contour, 1%. Peak, 7.9 Jy/beam. Beam, 3.5 0.7 mas @ 4.2 .
From Figure 4.12, the core dimensions and flux density, de-convolved from
the restoring beam, are < 0.5 <0.5 mas and 9.4 Jy.
These values give an observed radio core brightness temperature of > 2.1 10 
K at the frequency of 4.8 GHz, corresponding to a source frame brightness temperature of 
K. Thus PKS 1921-293
possibly has the highest brightness temperature of the seven radio
sources considered here, in excess of the inverse Compton limit for
synchrotron radiation. This result is consistent with the findings of
Mollenbrock et al. [1996] who measured 140 extragalactic radio
sources with trans-continental baselines at a frequency of 22 GHz.
They found that PKS 1921-293 had the highest correlated flux density
and observed brightness temperature in their sample, 12.6 
Jy and
>7 
10 K respectively.
The high brightness temperature inferred from ground-based VLBI
observations [this work; Mollenbrock et al., 1996] is confirmed
from VLBI observations with greater than Earth diameter baselines. From
the Tracking and Data Relay Satellite System (TDRSS) space VLBI experiments Linfield et al. [1989] found
that PKS 1921-293 had a source frame brightness temperature of approximately 3.8
10 K at a frequency of 2.3 GHz.