All Figures

$\begin{figure} \par\includegraphics[width=12cm,clip]{0021f1.eps}\end{figure}$ Figure 1: Map of near-infrared brightness ( $\lambda 1.6~\mu$ m), rotated with position angle $222\hbox {$^\circ $ }$ along the horizontal. a) Sum of images with total exposure time 23 040 s, showing the diffraction spike passing through part of the jet. b) Sum of remaining images with total exposure time 11 520 s, diffraction spike clear of the jet. c) as a, after subtraction of diffraction spike modeled on b).
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In the text

$\begin{figure} \includegraphics[width=13cm,clip]{0021f2.eps}\end{figure}$ Figure 2: Photometry of the jet in 3C 273 at 0 $.\!\!^{\prime\prime}$ 3 effective beam size. Clipped to show only measurements with aperture signal-to-noise ratio >5. Grey levels runs from 0 to the peak flux/beam with a pseudo-logarithmic stretch as indicated by the greyscale bar. Jet features are labelled as in Fig. 1. The offset of 0 $.\!\!^{\prime\prime}$ 2 between radio and optical hot spot position can be made out clearly.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{0021f3.eps}\end{figure}$ Figure 3: Plot of surface brightness per beam along the jet's ridge line, i.e., showing the brightest point per column from Fig. 2.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=14cm,clip]{0021f4.eps}\end{figure}$ Figure 4: Normalised cuts through the jet profile at various distances from the quasar at 300 nm (solid line), 620 nm (short dash), 1.6 $\mu$ m (long dash), and 2.0 cm (dotted); resolution is 0 $.\!\!^{\prime\prime}$ 3. Negative offsets are to the south of the jet. The shaded Gaussian in the first panel shows the resolution of 0 $.\!\!^{\prime\prime}$ 3 FWHM. The "southern extension'' is visible in the short-wavelength profiles at $r=12\hbox{$.\!\!^{\prime\prime}$ }8$ . However, the apparently corresponding feature in the radio profile is in fact unrelated. Instead, it is due to the radio cocoon around the jet: Röser et al. (1996, cf. their Fig. 3# noted that the radio emission is more extended, in particular to the south, than the optical emission. There is a similar tendency for the near-infrared profile to be more extended than the optical/UV. Other than that, the profiles show a similar jet width at all wavelengths. The cut through B1 clearly shows the transverse offset in optical and radio emission between the jet's two strands. In D2, the radio peak appears offset slightly to the south from the optical peak, but the interpretation of this shift is uncertain.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0021f5.eps}\end{figure}$ Figure 5: Comparison of hotspot position at optical and radio wavelengths. Greyscale shows the 620 nm image at original resolution, contours show the 1.3 cm radio map at 0 $.\!\!^{\prime\prime}$ 125 resolution. Coordinates are relative to the quasar core, using which both images were aligned. The radio hot spot is clearly offset by 0 $.\!\!^{\prime\prime}$ 2 from the optical hot spot, while the radio contours closely agree with the optical image in the preceding region (D2/H3).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0021f6.eps}\end{figure}$ Figure 6: Comparison of jet full-width at half the maximum intensity at different wavelengths. The width is determined column by column on the images in Fig. 2 as the full width at half the maximum intensity along the column. The bottom panel shows the optical ( $\lambda$ 620 ${\rm nm}$ ) flux profile (maximum intensity) for reference. The middle panel shows the width at $\lambda$ 300 nm (UV), $\lambda 620$ ${\rm nm}$ (optical) and $\lambda 1.6~\mu$ m (IR). The upper panel shows again the optical width for reference and the width at the radio wavelengths of $\lambda 1.3$ cm and $\lambda 2.0$ cm. The right-hand-side axis for the upper two panels expresses the observed width in units of the effective resolution of 0 $.\!\!^{\prime\prime}$ 3. This plot shows that although the jet is clearly wider in the radio than at HST frequencies, the increasing isophotal width in the radio is mainly caused by the increase in brightness, not by an actual widening, which would be seen as an increase in the jet's FWHM.
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In the text

$\begin{figure*} \par\hspace*{2.45cm}\parbox[c]{0.1\hsize}{\hspace*{0.1\hsize}}\p... ...0.79\hsize}{\includegraphics[width=0.79\hsize,clip]{0021f7m.eps}} \end{figure*}$ Figure 7: Spectral index maps at 0 $.\!\!^{\prime\prime}$ 3 resolution generated from the photometry data in Fig. 2. Only pixels with a signal-to-noise ratio of at least 5 per beam are shown. Images are combined pairwise in order of increasing wavelength. Linear colour scales (shown above the respective images) have been chosen to stress variations within one map. a) Optical spectral index (range: -2...0); b) optical-infrared (-2.5...-0.4); c) infrared-radio (-1.35...-0.8); d) radio $\lambda 1.3$ cm- $\lambda 2.0$ cm (-2...0); e) radio $\lambda 2.0$ cm- $\lambda 3.6$ cm (-2...0). The variations of both radio spectral indices in the inner part of the jet are mainly due to low signal-to-noise and the associated imaging uncertainties. Compare with Fig. 8 to gauge the relative magnitude of variations of the different spectral indices.
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In the text

$\begin{figure} \par\includegraphics[origin=rB,angle=270,width=12cm,clip]{0021f8.eps} \end{figure}$ Figure 8: Run of the spectral indices along the jet at 0 $.\!\!^{\prime\prime}$ 3 resolution, sampled in 0 $.\!\!^{\prime\prime}$ 1 intervals (cut along radius vector at position angle 222 $.\!\!^\circ$ 2). For sake of clarity, only typical $2\sigma$ error bars are shown for the random error. Systematic flux calibration uncertainties are of the same order and would shift an entire curve. The radio spectral index $\alpha _{{\rm radio}}$ is obtained by a fit to the radio data at 3.6 ${\rm cm}$ , 2.0 ${\rm cm}$ , and 1.3 ${\rm cm}$ . The other spectral indices are derived from the jet photometry at the given wavelengths ( $\alpha _{1.3}^{{\rm IR}}$ : 1.3 ${\rm cm}$ and 1.6 $\mu$ m, $\alpha ^{{\rm opt}}_{{\rm IR}}$ : 1.6 $\mu$ m and 620 ${\rm nm}$ , $\alpha ^{{\rm UV}}_{{\rm opt}}$ : 620 ${\rm nm}$ and 300 ${\rm nm}$ ). The optical flux profile is shown for reference. Reprinted from Jester et al. (2002) for reference.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{0021f9.eps}\end{figure}$ Figure 9: Comparison of high-frequency spectral indices with previous observations at 1 $.\!\!^{\prime\prime}$ 3 resolution. Top: comparison of optical spectral index $\alpha _{{\rm BRI}}$ at 1 $.\!\!^{\prime\prime}$ 3 resolution from Röser & Meisenheimer (1991) with optical-UV spectral index $\alpha ^{{\rm UV}}_{{\rm opt}}$ at 0 $.\!\!^{\prime\prime}$ 3 from this work (error bars are similar for $\alpha _{{\rm BRI}}$ and $\alpha ^{{\rm UV}}_{{\rm opt}}$ ). Below: comparison of infrared-optical spectral index $\alpha _{{\rm KO}}$ from Neumann et al. (1997a) and $\alpha ^{{\rm opt}}_{{\rm IR}}$ from this work (only every tenth error bar shown). The overall agreement between the spectral indices at 0 $.\!\!^{\prime\prime}$ 3 and 1 $.\!\!^{\prime\prime}$ 3 is surprisingly good. Differences between $\alpha ^{{\rm UV}}_{{\rm opt}}$ and $\alpha _{{\rm BRI}}$ occur in A, C2, D1, and D2, those regions in which the observed spectrum (Fig. 8) does not show the expected steepening towards higher frequencies.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{0021f10.eps}\end{figure}$ Figure 10: Illustration of the two different spectral fits performed. Model A assumes infrared emission in excess of the cutoff given by the optical-ultraviolet spectral index, while model B assumes an ultraviolet excess above the infrared-optical cutoff.
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{0021f11a.eps}}\vspace*{4mm} \includegraphics[width=7.45cm,clip]{0021f11b.eps}\end{figure}$ Figure 11: Location of apertures for which spectra are shown in Fig. 12. Locations at local peaks ("knots'') are marked by a line below the image, while locations in inter-knot regions are marked by a line above.
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In the text

$\begin{figure} \par\includegraphics[width=7.05cm,clip]{0021f12a.eps}\hspace*{3mm... ....eps}\hspace*{3mm} \includegraphics[width=7.05cm,clip]{0021f12f.eps}\end{figure}$ Figure 12: Observed data points with fitted spectra for the points shown in Fig. 11. To account for the observed flattening of the spectrum towards the ultraviolet, model A assumes a contamination in the infrared, so that the cutoff is determined by the optical-ultraviolet spectral index there, while the cutoff in model B is determined by the infrared-optical spectral index. Those spectra which require an artificial high-frequency data point to obtain a fit result are labelled "cut''; for these, models A and B are identical, and the artificially obtained value for $\nu _{\rm c}$ is a lower limit to the actual value. The spectrum may extend up to X-rays in those locations where an artificial cut is necessary (cf. Marshall et al. 2001; Röser et al. 2000; Jester et al. 2002).
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In the text

$\begin{figure} \par\includegraphics[width=7.05cm,clip]{0021f13a.eps}\hspace*{3mm... ....eps}\hspace*{3mm} \includegraphics[width=7.05cm,clip]{0021f13f.eps}\end{figure}$ Figure 12: continued.
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In the text

$\begin{figure} \par\includegraphics[width=7.05cm,clip]{0021f14a.eps}\hspace*{3mm... ....eps}\hspace*{3mm} \includegraphics[width=7.05cm,clip]{0021f14d.eps}\end{figure}$ Figure 12: continued.
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In the text

$\begin{figure*} \par\hspace*{0.2mm}\parbox[c]{.15\hsize}{Mod.\ A\\ and HS}\parbo... ...ox[c]{7cm}{\includegraphics[angle=0,width=7cm,clip]{0021f15e.eps}} \end{figure*}$ Figure 13: Maps of the cutoff frequency. Grey levels run from 10¹³ Hz (white) to 10¹⁷ Hz (black) with a pseudo-logarithmic stretch as indicated by the greyscale bar. The values fitted in A and B2 are lower limits. As expected, model A bears a closer resemblance to the optical-ultraviolet spectral index map, while model B is dominated by the infrared-optical spectral index map (cf. Fig. 7). Regions H2 and H1 are fitted with a different model HS.
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In the text

$\begin{figure} \par\includegraphics[width=7.85cm,clip]{0021f16.eps} \end{figure}$ Figure 14: Run of the fitted cutoff frequency $\nu _{\rm c}$ along the jet. The thin line represents the run of the cutoff frequency at 1 $.\!\!^{\prime\prime}$ 3 resolution, parameterised as $\mbox{$\nu_{\rm c}$ }= 10^{17}~{\rm Hz} \exp(-(r-12\hbox{$^{\prime\prime}$ })/1\hbox{$.\!\!^{\prime\prime}$ }4)$ by Meisenheimer et al. (1996a). Model A has a slightly higher $\nu _{\rm c}$ than model B. Lower limits to the true cutoff frequency are indicated by arrows. Error bars are not shown because the errors on the fitted cutoff frequency are correlated in a complicated manner with the observational flux errors and the assumed spectral shape. Variations in the cutoff frequency are expected to be significant where the variations in the high-frequency spectral indices are significant.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=7.8cm,clip]{0021f17.eps} \end{figure}$ Figure 15: Run of the minimum-energy field $B_{{\rm min}}$ along the jet. The solid points show the values determined for individual regions by Meisenheimer et al. (1996a). The overall run corresponds to that of the jet luminosity. The most recent value for the magnetic field determined for the hot spot is (39⁺²⁴_-10) nT; correcting for a different value of k in Eq. (7) brings this into agreement with the present determination. Other discrepancies are due to the different volumes assumed for the emission regions, which were modeled as individual blobs by Meisenheimer et al. (1996a).
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{0021f18a.eps}\vspace*{1mm}... ...ar {\hspace*{4cm}radius/\hbox{$^{\prime\prime}$ }\hspace*{4cm}} \par\end{figure}$ Figure 16: Map of the bolometric surface brightness assuming isotropic emissivity. Values range from 1.5 $\times$ 10³⁴ W/pixel at the peak of knot A to 1.9 $\times$ 10³⁵ W/pixel in the hot spot with a pseudo-logarithmic stretch as indicated by the greyscale bar. The hot spot has a luminosity of approximately 1.31 $\times$ 10³⁷ W (sum of H2 and H1), which is just a little more than the total luminosity of the jet at $r\leq 20\hbox{$.\!\!^{\prime\prime}$ }7$ of 1.26 $\times$ 10³⁷ W. The luminosity of regions A and B2 could be much larger than indicated here if the synchrotron spectrum extends up to X-rays.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{0021f19.eps} \end{figure}$ Figure 17: Run of the maximum particle Lorentz factor $\gamma _{{\rm max}}$ along the jet. The thin line represents the run parameterised as $\mbox{$\gamma_{{\rm max}}$ }=$ $10^{7} \exp(-(r-12\hbox{$^{\prime\prime}$ })/2\hbox{$.\!\!^{\prime\prime}$ }4)$ by Meisenheimer et al. (1996a). Their points lie above our new The overall run of $\gamma _{{\rm max}}$ is identical to that of $\nu _{\rm c}$ .
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In the text

$\begin{figure*} \par\parbox[c]{.15\hsize}{image\\ 620~\mbox{${\rm nm}$ }}\parbox... ...hsize}{!}{\includegraphics[clip,width=.9\hsize]{0021f20c.eps}}}\\ \end{figure*}$ Figure 18: Map of the maximum particle Lorentz factor $\gamma _{{\rm max}}$ . The results for model HS have been inserted in both. Only in regions A and B2 does $\gamma _{{\rm max}}$ show a strong correlation to features of the radio-optical jet morphology. The only differences between models A and B occur in regions C-H3, where the discrepancies between the infrared-optical and optical-ultraviolet spectral indices are strongest.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{0021f3.eps}\end{figure}$	Figure 3: Plot of surface brightness per beam along the jet's ridge line, i.e., showing the brightest point per column from Fig. 2.
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{0021f10.eps}\end{figure}$	Figure 10: Illustration of the two different spectral fits performed. Model A assumes infrared emission in excess of the cutoff given by the optical-ultraviolet spectral index, while model B assumes an ultraviolet excess above the infrared-optical cutoff.
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$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{0021f11a.eps}}\vspace*{4mm} \includegraphics[width=7.45cm,clip]{0021f11b.eps}\end{figure}$	Figure 11: Location of apertures for which spectra are shown in Fig. 12. Locations at local peaks ("knots'') are marked by a line below the image, while locations in inter-knot regions are marked by a line above.
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$\begin{figure} \par\includegraphics[width=7.05cm,clip]{0021f13a.eps}\hspace{3mm... ....eps}\hspace{3mm} \includegraphics[width=7.05cm,clip]{0021f13f.eps}\end{figure}$	Figure 12: continued.
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