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

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5. Conclusions

We have obtained infrared K -band imaging of 11 hot spots in the lobes of powerful radio galaxies. We have detected infrared counterparts for three hot spots (3Cs 20W, 33S, 111E) which have known optical counterparts. The optical candidate in the western lobe of 3C 303 was also detected and shows a K-band morphology which resembles the radio morphology of the southern subcomponents. The K-band detection of the radio jet and the steep radio-to-optical spectrum of the hot spot complex point to it being the brightest knots in the jet rather than a genuine hot spot. Two hot spots in the powerful radio source Cygnus A (3C405) are found to be confused by galactic stars (hot spots A and B) and the third hot spot (D) was undetected. No infrared counterparts were found for the hot spots 3C 20 E, 65, and 123 E. We have used these detections and the upper limits for the brightest hot spots in Cygnus A to derive the overall synchrotron spectra from radio to optical frequencies.

Adding in the results for the leading hot spots of 3C 273 A and Pictor A (west) we now have collected reliable synchrotron spectra for 8 hot spots. Six of them show typical hot spot spectra which are characterized by a low frequency power-law [FORMULA] and a high frequency cutoff in the range [FORMULA], that is well below the value found in the optical counterparts of radio jets (Meisenheimer et al. 1996a, b). Three of them (3C 273 A, Cygnus A, hot spots A & D) show clear evidence for a spectral break by [FORMULA] in the radio regime ([FORMULA]) which is expected if a thin acceleration region (strong shock) is followed by an extended downstream emission region in which synchrotron losses dominate (Heavens & Meisenheimer 1987). In all six cases the standard scenario of diffusive shock acceleration at the strong terminal shock provides a self-consitent description even for mildly relativistic jet speeds [FORMULA].

However, the steep and over a wide area constant synchrotron spectra found in 3C 33 S, 3C 303 W and Pictor A (west) cannot easily be understood in this popular model. In addition, these hot spots exhibit exeptionally high cutoff frequencies [FORMULA] Hz which place them right in the middle of the distributions of spectral indices and cutoff frequencies observed in optically detected radio jets. The extraordinary spectra and the absence of any indication of synchrotron ageing in these hot spots led us to the speculation that there exists a second "jet-like" acceleration process which is responsible for the synchrotron spectra of both optical jets and optically extended radio hot spots. The latter acceleration process has to be very efficient (typical acceleration time scale [FORMULA] years around [FORMULA]) in order to replenish the particle energy losses almost "on the spot". Although its physics is completely unkown at present, we feel that a better understanding of particle acceleration and synchrotron spectra of extended radio source in general will be impossible unless we have an idea how this new process works.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998