PKS 0438-436 is one of the bright, flat-spectrum radio sources
considered by von Montigny et al. [1995b] to be of interest as a blazar not identified by EGRET. von Montigny et al. [1995b] place an
upper limit of 3.4 
10 
cm 
s 
for greater than 100 MeV emission from PKS 0438-436. PKS 0438-436 is one of the highest red shift, and
consequently highest luminosity, radio loud quasars at z=2.852. It
exhibits the typical blazar characteristic of high and variable optical
polarisation [Impey & Tapia 1988].
Figure: Low contour, 
1%. Peak, 1.6 Jy/beam. Beam, 3.2 2.6 mas @ -13.8 
The 1982 VLBI observations at 2.3 GHz of Preston et al. [1989] with
the SHEVE array, were the first of PKS 0438-436 and were used to
produce both a model and an image. It was found that the compact
source consisted of two 1.9 Jy components separated by 35 mas at a
position angle of -43 , with the south-east component slightly
more extended than the north-west component. They also found a large
position angle difference between the VLBI structure and a 2''
extension at a position angle of approximately 10 observed
with the VLA [Perley 1985]. The angular length and position angle of
this arcsecond extension has been confirmed with two-frequency observations at
the ATCA [Lovell, McCulloch, & Jauncey 1995].
Images obtained, at a frequency of 2.3 GHz, in 1989 showed no significant change in the structure of the radio source, although both components remained essentially unresolved at this resolution [Murphy et al. 1993].
The new VLBI data, at the higher frequency of 4.8 GHz (Figure 4.2)
show the two components seen at lower frequency. The south-east
component is less compact than the flat-spectrum north-west component, which is the core of the radio source. The two components are both
elongated, but not along the same position angle. The north-west
component has a position angle of approximately 124 whereas the
south-east component has a position angle of approximately
35 . The position angle connecting the centroids of the two
components is approximately 136 , in good agreement with the
results of Preston et al. [1989] and Murphy et al. [1993].
The distance between the centroids of the two components is 36.2 mas,
only 1.2 mas greater than in 1982. The mismatch in frequency and
resolution between the observations do not allow an accurate estimate
of any apparent speed of separation between these components.
From Figure 4.2, the core FWHM and integrated flux density have been
estimated to be 0.6 0.2 mas and 1.6 Jy respectively. The
observed radio core brightness temperature is therefore approximately 6.0
10 
K at 4.8 GHz, corresponding to approximately 
K in the source frame, at or above the inverse Compton limit for synchrotron radiation.
The misalignment between the VLBI and VLA radio structures noted by
Preston et al. [1989] is continued on the mas-scale between the
two compact components. The misalignment between the
mas-scale and arcsecond-scale structures is approximately
126 . The misalignment on the mas-scale, between the two components, is approximately 89 .