Jet spectra in FR I radio galaxies: implications for particle   acceleration
  We describe very accurate imaging of radio spectral index for the inner jets in three FR I radio galaxies. Where the jets first brighten, there is a remarkably small dispersion around a spectral index of 0.62. This is also the region where bright X-ray emission is detected. Further from the nucleus, the spectral index flattens slightly to 0.50 - 0.55 and X-ray emission, although still detectable, is fainter relative to the radio. The brightest X-ray emission from the jets is therefore not associated with the flattest radio spectra, but rather with some particle-acceleration process whose characteristic energy index is 2.24. The change in spectral index occurs roughly where our relativistic jet models require rapid deceleration. Flatter-spectrum edges can be seen where the jets are isolated from significant surrounding diffuse emission and we suggest that these are associated with shear. 
R. A. Laing (1), A. H. Bridle (2), W. D. Cotton (2), D. M. Worrall   (3), M. Birkinshaw (3) ((1) ESO Garching, (2) NRAO Char...