The Structure of the Gravitational Lens System B1152+199
P.G. Edwards , J.E.J. Lovell , H. Hirabayashi , D.L. Jauncey S. Toft, PASA, 18 (2), in press.

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Full-synthesis ATCA Image

We undertook a full-synthesis, polarimetric imaging observation with the ATCA at 6.1 and 8.6 GHz on 24 June 2000 to obtain a ``template'' model for use in model-fitting the snap-shot observations, following the method successfully employed in determining the delay of the gravitational lens PKS 1830-211 (Lovell et al. 1998). The resulting 8.6 GHz image of the central region is shown in Figure 1, which reveals that the two lensed images of the quasar can be clearly distinguished. The results of model-fitting the two components are given in Table 1. The spectral index between 6.1 and 8.6 GHz, of $\alpha$ $\sim$ -0.3 (S $\propto$ $\nu^\alpha$ ) is the same as that found by Myers et al. (1999) between 8 and 15GHz. These observations do not have the resolution of the VLA A-configuration observations of Myers et al. (1999), however for the purposes of monitoring, the ATCA has the advantage over the VLA of providing regular access to $\sim$ 6km baselines throughout the year, yielding the required arcsecond resolution at 8.6GHz. In contrast, for half the year the VLA has a maximum antenna spacing of 3.6km or less which is insufficient to resolve the components of the lens.

**Figure 1:** June 2000 ATCA image of B1152+199 at 8.6GHz. The ATCA image is consistent with the two point-like images of the lensed quasar separated by 1.56'', as discovered by Myers et al. (1999). Polarimetry revealed a high rotation measure towards the weaker component south-east (i.e. to the lower left) of the brighter component at the origin of the figure. The map peak is 48 mJy/beam, and contour levels are -0.25, 0.25, 0.5, 1, 2, 4, 8, 16, and 32 mJy/beam. The beam (FWHM) is 3.2arcsec x 0.8arcsec at a position angle of 0.2 $^\circ$ .
$\begin{figure} \begin{center} \centerline{\psfig{file=Edwards_fig1.ps,height=7cm,angle=270}}\end{center}\end{figure}$

Polarized flux was detected from the lensed components. The polarization fraction is preserved in gravitational lensing (e.g. Schneider, Ehlers, & Falco 1993). The percentage polarization of component A is (2.7 $\pm$ 0.1)% at 6.1GHz and (2.4 $\pm$ 0.2)% at 8.6GHz. For component B the percentage polarization is (1.9 $\pm$ 0.4)% at both frequencies. Combination of the polarimetric data from both frequencies allowed the rotation measures to be estimated (with inherent phase-turn ambiguities). Component A has a rotation measure of $\sim$ 70rad/m² and component B has a rotation measure of (at least) $\sim$ 560rad/m². The errors are appreciable, $\sim$ 100% for component A and $\sim$ 20% for component B, but it is clear there is a much larger rotation measure towards component B. The implication is that the radiation forming the southern image of the background quasar passes much closer to the center of the foreground lensing galaxy.

This was confirmed by recent UBVRIz' photometry of the system with the 2.56m Nordic Optical Telescope (NOT) by Toft, Hjorth, & Burud (2000), in which both lensed images and the lensing galaxy were detected. The position of the lensing galaxy was found to be almost coincident with the fainter quasar image, component B. Component B suffers heavy extinction, relative to the brighter quasar image, by dust in the lensing galaxy. As component A appears to suffer little extinction, Toft et al. measured the relative intensity ratios in the different bands to determine the extinction curve of the lensing galaxy.

The other result from our full ATCA synthesis was the detection at 6.1GHz of a fourth source (``component D'') in the field, on the opposite side of component C, as shown in Figure 2. The most natural interpretation from the alignment of these components is that C and D are hotspots in low surface brightness radio lobes. Component C has a peak flux density at this frequency of 1.9mJy/beam, whereas that of component D is 0.3mJy/beam (with a noise level of $\sim$ 0.05mJy/beam). Component D is thus both fainter and further from the center of the galaxy than component C. This is in contrast to expectations from the symmetric relativistic model of Ryle and Longair (1967), in which structural asymmetry is due solely to differences in light travel time. This model predicts that the fainter component would be closer to the core. However, as discussed by Arshakian and Longair (2000), the simple model excludes important environmental asymmetries and (probably less significant) intrinsic jet asymmetries, and a better understanding of such systems can be obtained by adopting an asymmetric relativistic model. We are planning further multi-frequency observations to unambiguously determine the rotation measures to the lensed images and, through higher sensitivity imaging, to study the spectral properties of the two radio lobes in more detail.

**Figure 2:** ATCA image of B1152+199 at 6.1 GHz. Component C, first detected by Myers et al. (1999), is to the right of the bright lensed components and component D, discovered in this June 2000 observation, is to the left. Component C was also detected in the 8.6 GHz ATCA observation, but component D was below the sensitivity limit. Contour levels are -0.25, 0.25, 0.5, 1, 2, 4, 8, 16, 32, and 64% of the map peak of 59mJy/beam. The beam (FWHM) is 7.0arcsec x 1.5arcsec at a position angle of 0 $^\circ$ .
$\begin{figure} \begin{center} \centerline{\psfig{file=Edwards_fig2.ps,height=5cm,angle=270}}\end{center}\end{figure}$

Although components C and D appear extended, we examined the corresponding positions on the NOT image to see if there were optical counterparts. To our surprise, there was an I $\sim$ 18 star-like object in the vicinity of component C. Preliminary analysis of follow-up VLT observations, however, reveals that the optical object is offset by several arcseconds from the centroid of the radio emission, and furthermore, that it has a rather red color, typical (within photometric uncertainties) of an M-type star, indicating that the optical object is a foreground star. There was no object visible in the NOT image in the vicinity of component D.

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Full-synthesis ATCA Image