5 Discussion

Due to the non-detection of polarized emission at 320MHz in both 0235-197 and 1203+043, we cannot explain the low frequency variability observed with the Molonglo Cross (McAdam 1980) in terms of ionospheric Faraday rotation as in the case of 3C 159.

Can refractive scintillation (Rickett 1986) explain the variability of 0235-197? The refractive scintillation models assume a supposedly-variable radio source to have most of its flux density in a single compact component. The degree of variability is a function of the characterization of the interstellar medium (itself a function of galactic coordinates) and source size. 0235-197 is at galactic latitude $\vert b\vert=65^{\circ}$ and was reported to vary (rms variability $\sim 0.2$ Jy) on a time scale of the order of 1 year (McAdam 1980).

In the usual refractive model of interstellar turbulence (Mantovani et al. 1990b; Spangler et al. 1993; Spangler et al. 1994; Bondi et al. 1994), the relevant parameter for the observed scintillation index is $\theta_{\rm eff}^{7/6} \sqrt{\sin b/I}$, where $\theta_{\rm eff}=\theta_{\mathrm{FWHM}}/2.35$ and I is the parameter indicator of the source structure (=1 for a Gaussian structure). In such a model a source of ($\approx 3$Jy) with a rms variability of 0.2Jy and corresponding scintillation index of $\sim 0.06$ should have an angular diameter in the range 10-20 mas (see Spangler et al. 1993 for details).

The $E_{\rm 1a}$ hot spot has a spectrum which, extrapolated towards lower frequencies, gives a mean flux density of $\sim$3 Jy at 408 MHz. This is comparable to the peak flux density found at 320 MHz. The crucial parameter is, however, the angular size of the hot spot. The deconvolved size found for the $E_{\rm 1a}$ hot spot at 15 GHz is <0.2 arcsec. Even if in principle there is not contradiction, the scintillation theory requires an angular size for the hot spot in 0235-197 that is a factor 7-10 smaller than the size measured at 15GHz, which is close to the sizes usually measured for the hot spots. Low frequency VLBI observations are needed to confirm the existence of such a compact component in the hot spot.

The lack of polarized emission at 320MHz for 1203+043 and a radio structure which lacks a bright compact component rules out both of the mechanisms for low frequency variability in this source. Consequently, we conclude that this source might be a spurious case of variability. However, 1203+043 has an interesting structure at 320MHz. It shows a pair of secondary lobes in a direction perpendicular to the main source axis, making the object one of a few known "X''-shaped sources. At present, only about ten sources are known to show such morphology. They are believed to have both young and old lobes. These lobes may be supplied by jets whose direction has changed with time. A change in the orientation of the central engine due to precession has been suggested by Ekers et al. (1978) for NGC 326 to account for its "X'' shaped morphology. Such a model has been applied successfully to 0828+32 by Klein et al. (1995) but with the extra assumption that the length of the precessing beam changes with time. A merger between galaxies is thought to be the cause of the precession. However, Ulrich-Demoulin & Rönnback (1996) have reported that optical images of 0828+32 do not show the signature of a recent major merger event.

However, the structure of 1203+043 looks peculiar when compared with other "X'' shaped sources. For example, it has an asymmetric structure with respect to the component C which is believed to be the core (Figs. 7 and 6 in Mantovani et al. 1992). The long, bent jet is clearly "one-sided'' while, generally speaking, the members of the class show two-sided jets (at the available resolution). Moreover, the young lobes of the "X'' shaped sources are dominated by hot spots while here the jet emission fades away from the core and the northern lobe contains only diffuse emission without any bright component. This asymmetry is reflected in the 320 MHz map where the region to the North-West is more extended and brighter than the opposite side.