1 Introduction

While radio jets are a common feature of radio galaxies and quasars, optical emission has to date been observed from only about 15 extragalactic jets. As shown by polarimetric observations (starting with Baade 1956 for M 87), both the radio and optical emission is synchrotron continuum radiation. While information on the source's magnetic field structure may be obtained from the polarisation structure, the diagnostic tool for the radiating particles is a study of the synchrotron continuum over as broad a range of frequencies as possible, i.e., from radio to UV or even X-ray wavelengths, and with sufficient resolution to discern morphological details. The radio and optical emission observed from hot spots in radio jets can be well explained by first-order Fermi acceleration at a strong shock in the jet (the bow shock) (Meisenheimer & Heavens 1986; Heavens & Meisenheimer 1987; Meisenheimer et al. 1989, 1997). But it is not clear that the optical synchrotron emission from the jet body, extending over tens of kiloparsecs in some cases, can be equally well explained by acceleration at strong shocks inside the jet. As is well known from standard synchrotron theory, electrons with the highly relativistic energies required for the emission of high-energy (optical and UV) synchrotron radiation have a very short lifetime which is much less than the light-travel time down the jet body in, e.g., 3C273. Observations of optical synchrotron emission from such jets (Röser & Meisenheimer 1991; 1999) as well as from the "filament'' near Pictor A's hot spot (Röser & Meisenheimer 1987; Perley et al. 1997) suggest that both an extended, "jet-like'' and a localized, "shock-like'' acceleration process are at work in these objects in general and 3C273's jet in particular (Meisenheimer et al. 1997). The extended mechanism may also be at work in the lobes of radio galaxies, where the observed maximum particle energies are above the values implied by the losses within the hot spots (Meisenheimer et al. 1996) and by the dynamical ages of the lobes (Blundell & Rawlings 2000). The fundamental question is thus: how can we explain high-frequency synchrotron emission far from obvious acceleration sites in extragalactic jets? Although most of the known optical jets are very small and faint (Scarpa & Urry 2000), there are a few jets with sufficient angular size and surface brightness to be studied in detail: those in M 87 (a radio galaxy), PKS  0521-365 (an elliptical galaxy with a BLLac core), and 3C273 (a quasar). We have embarked on a detailed study of the jet in 3C273 using broad-band observations at various wavelengths obtained with today's best observatories in terms of resolution: the VLA (in combination with MERLIN data at $\lambda$6cm) and the HST. Using these observations, we will derive spatially resolved (at 0 $.\!\!^{\prime\prime}$2) synchrotron spectra for the jet. 3C273's radio jet extends continuously from the quasar out to a terminal hot spot at 21 $.\!\!^{\prime\prime}$5 from the core, while optical emission has been observed only from 10 $^{\prime \prime }$ outwards. On ground-based images, the optical jet appears to consist of a series of bright knots with fainter emission connecting them. So far, synchrotron spectra have been derived for the hot spot and the brightest knots using ground-based imaging in the radio (Conway et al. 1993), near-infrared $K^{\prime}$-band (Neumann et al. 1997) and optical I, R, B-bands (Röser & Meisenheimer 1991) at a common resolution of 1 $.\!\!^{\prime\prime}$3 (Meisenheimer et al. 1996a; Röser et al. 2000). The radio-to-optical continuum can be explained by a single power-law electron population leading to a constant radio spectral index of -0.8, but with a high-energy cutoff frequency decreasing from $10^{17}\,$Hz to 10¹⁵Hz outwards along the jet. The aim of the study is both the determination of the spectral shape of the synchrotron emission, and by fitting synchrotron spectra according to Meisenheimer et al. (1989), deriving the maximum particle energy everywhere in the jet. The observed spectra can then be compared to predictions from theoretical work.

 

 

Table 1: Passbands and limiting magnitudes for the observations. Point source: $10\sigma$ detection limit, prediction by the exposure time calculator. Extended source: $5\sigma$ per pixel detection limit, determined from the background noise measured on reduced frames. Magnitudes are Vegamags referred to the corresponding HST filter band.

Filter	Mean $\lambda$	FWHM	Exposure	Point source		Extended source
	nm	nm	s	mag	$\mu$Jy	mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\hbox{$^{\prime\prime}$ }$	$\mu$Jy/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\hbox{$^{\prime\prime}$ }$
F300W	301	77	35500	26.1	0.04	20.8	5.2
F622W	620	92	10000	27.7	0.04	23.0	2.8

As an intermediate result of our study, we present HST WFPC2 images of the jet in 3C273 in a red (F622W) and near-UV (F300W) broadband filter. The near-UV observations constitute the highest-frequency detection of synchrotron emission from 3C273 so far. (Extended X-ray emission has also been observed, but it is unclear at present whether this, too, is due to synchrotron radiation, Röser et al. 2000; Marshall et al. 2001; Sambruna et al. 2001.) From these images, we construct an optical spectral index map at 0 $.\!\!^{\prime\prime}$2 resolution. After a description of the observations and the data reduction in Sects. 2 and 3, we examine the direct images in Sect. 4. The creation and description of the spectral index map follow in Sect. 5. We analyse the map in Sect. 6 and conclude in Sect. 7. Details regarding the alignment of HST images are found in Appendix A.