Overlay of 3C66B X-ray contours (in 0.3-7.0 keV) on
HST optical image from STIS.

Note the excellent correspondance between the X-ray
and optical structures: unlike 3C15, the optical and X-ray
features in this object seem to correspond closely. This
suggests a synchrotron, rather than inverse-Compton,
origin for the radiation (supporting the analysis of
Hardcastle et al.). The short lifetime of X-ray emitting
electrons in the jet then proves that in situ particle
acceleration is needed.

Note that even the outermost, faint, optical feature on the
image has a strong X-ray counterpart.