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Astron. Astrophys. 325, 57-73 (1997)

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1. Introduction

This paper presents infrared imaging observations of a sample of hot spots in the lobes of powerful, low-redshift, classical double radio sources, and is thus an extension of our multi-wavelength imaging and polarimetry observations of hot spots in radio sources (Meisenheimer et al. 1989, hereafter Paper I). Hot spots are frequently found near the outermost edges of the lobes of powerful, classical FRII radio sources, and are characterised by their high radio surface brightness. In a seminal paper, Blandford & Rees (1974) explained these hot spots as the "working surfaces" of supersonic jets in which the jet's kinetic energy is partly dissipated into the acceleration of relativistic particles, giving rise to the observed intense synchrotron radiation. Their hypothesis was later impressively confirmed by several cases in which narrow radio jets connecting the core of the radio source with the hot spot have been observed (e.g. Cygnus A, 3C 111, Pictor A).

The first conclusive evidence that the synchrotron spectra of these hot spots extend up to optical frequencies was provided by Meisenheimer & Röser (1986) who showed that the optical counterpart to the radio hot spot 3C 33 South is strongly polarized (29%). Subsequent work has concentrated on the acquisition of data for further hot spots over as large a wavelength range as possible and the first results of this multi-wavelength campaign were presented in Paper I. This multi-wavelength approach is motivated by the need to obtain an accurate measure of the shape of the spectrum so as to constrain the nature of the acceleration mechanism which is able to produce electrons at ultra-relativistic energies emitting the observed synchrotron light. Paper I showed that the infrared data are of great importance in determining the shape accurately, especially given the wide gap between the radio and optical parts of the spectra and presented firm K -band detections of two hot spots (3C 33 S and 3C 111 E) and an upper limit for 3C 123 E (all obtained with aperture photometry). This work presents new infrared imaging for these three hot spots in order to improve on their photometry as well as infrared imaging for a further eight examples.

Specifically, Paper I showed for the first time that the hot spot spectra could be accurately modelled by first-order Fermi acceleration at a strong shock. Our original aim has been to construct spectra for a larger number of hot spot candidates and to confirm the first order Fermi acceleration interpretation. However, in the last years there have been various observational indications that particle acceleration in extended radio sources is not confined to the hot spots or bright knots but might be distributed throughout a large fraction of the entire radio source (Neumann et al. 1995, Perley et al. 1997). Thus the hot spot spectra have to be discussed in this wider context.

Section 2 presents details of the observations, and Section 3 gathers together the available multi-wavelength data for each hot spot and presents their spectra. The new results are related to previous studies in Section 4 in order to derive general properties of the underlying particle acceleration mechanism.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998