Optical structure and physics of the M 87 jet

We summarize HST observations of the M 87 jet, concentrating on polarimetry and spectral index maps, and compare its optical and radio structures. The evidence now supports a stratified model for the structure of the jet, in which high-energy electrons emitting optical synchrotron radiation and their lower-energy, radio-emitting counterparts occupy separate regions of the jet, with different magnetic-field configurations. The higher-energy paxticles are closer to the jet axis, where the shocks that produce the knots in the inner jet appear to originate. Knot regions have optical spectra Which axe much flatter than the average for the jet, with the flattest-spectrum regions coinciding with flux maxima of the knots. These knots are preceded by regions where perpendicular apparent magnetic fields are seen. Thus not only do we see all the necessary ingredients for in situ particle acceleration in the knots, but there is now fairly direct evidence for it as well. By tracking the changes in radio-optical and optical spectral indices in the knot regions, we can estimate the relative acceleration. and cooling time-scales in the knots