1997A&A...325...57M

We present infrared imaging of a sample of 11 hot spots in the lobes of powerful radio galaxies. We have detected infrared counterparts to the hot spots in 3C 20 West, 3C 33 South, 3C 111 East and 3C 303 West all of which have known optical counterparts as well. In addition, we detected the radio jet of 3C 303 in the near infrared. Four other hot spots, those in 3C 20 East, 3C 65 (both sides), and 3C 123 East were not detected in the infrared. The optical candidates for the hot spots A and B in the western lobe of Cygnus A (3C 405) were visible in the infrared but their optical-infrared spectra reveal them both to be galactic stars. Neither infrared nor optical candidate was found for hot spot D in the eastern lobe of Cygnus A. We use the detections together with flux measurements at other frequencies to construct synchrotron spectra of the hot spots from the radio to the optical. Adding the bright hot spots in Cygnus A, the synchrotron spectra of which are well constrained by our upper limits and the millimetre flux, we are able to extend our general analysis of synchrotron spectra to 8 hot spots. We confirm our previous claim that first order Fermi acceleration could account for the particle acceleration in most hot spots but also provide evidence for our suspicion that the full range of physical parameters is much broader then our original sample indicated. However, in some well extended hot spots an additional acceleration process seems to be required.