Morphology

Images of the western hot spot at radio (PRM), optical (Röser 1989; PRM) and X-ray wavelengths with similar resolutions are shown in Fig. 3. As described in the Introduction, the radio and optical hot spots have a very similar morphology. In a higher resolution (0.''45 $\times$ 0.''09) radio image (Fig. 19 of PRM), the bright ``core'' is found to have a sharp leading (i.e. away from the nucleus) edge, with a slower decline on the side towards the nucleus. Behind this bright region is a linear feature elongated in p.a. 32$^{\circ }$ (Fig. 3), referred to as the ``filament'' by Röser & Meisenheimer (1987) and the ``plateau'' by PRM.

The overall X-ray morphology is remarkably similar to the radio and optical, with both the ``core'' and the ``filament'' detected. Much of the apparent elongation of the brighter part of the ``core'' of the X-ray hot spot (Fig. 3) reflects the shape of the point spread function (psf). Fig. 4 compares profiles along the major and minor axes of the hot spot through both the hot spot and the model psf at the location of the hot spot. At half maximum, the hot spot is somewhat wider than the model, but the model psf does not include broadening by imperfections in the aspect solution. In order to assess the reliability of the model psf, we have compared a compact X-ray source (visible in Fig. 1) some 152 $^{\prime\prime}$ from the nucleus in p.a. 40$^{\circ }$ with the Chandra model psf at this location. The observed profile of this source (which is at a similar distance from the aim point as the hot spot) is in good agreement with the model psf, but broader by 0.''2 at the half power point. This number is similar to the excess broadening of the hot spot over the model psf along its minor axis (Fig. 4), suggesting that the brighter part of the hot spot is unresolved in this direction. The excess FWHM of the hot spot over the psf along its major axis (Fig. 4) is somewhat larger. Nevertheless, we feel that caution is necessary, since other effects (e.g. the motion of the source on the detector during an observation) could lead to further broadening compared with the model psf. The conservative conclusion is that the brightest part of the ``core'' is marginally resolved or unresolved in X-rays.

There are, however, clear indications of structure in the hot spot at lower brightness levels. As in the radio, the X-ray core has a sharper edge away from the nucleus than towards it. A narrow ``ridge'' extends $\simeq$ 3.''5 (3.4 kpc) in p.a. 108$^{\circ }$ (the general direction towards the nucleus) from the hot spot peak. A weaker feature some 6 $^{\prime\prime}$ (5.7 kpc) SSW of the core and elongated towards p.a. 279$^{\circ }$, is seen in both the radio and X-ray images. In conclusion, when the small differences in resolution of the X-ray and radio images (Fig. 3) are taken into account, the X-ray and radio morphologies are remarkably similar. This morphological similarity indicates that the X-ray, optical and radio-emitting regions are physically co-spatial and argues that the X-ray emission does not originate from a bow shock ahead of the radio hot spot.