New Chandra observations of the jet in 3C273. I. Softer X-ray than radio   spectra and the X-ray emission mechanism
  The jet in 3C273 is a high-power quasar jet with radio, optical and X-ray emission whose size and brightness allow a detailed study of the emission processes acting in it. We present deep Chandra observations of this jet and analyse the spectral properties of the jet emission from radio through X-rays. We find that the X-ray spectra are significantly softer than the radio spectra in all regions of the bright part of the jet except for the first bright "knot A", ruling out a model in which the X-ray emission from the entire jet arises from beamed inverse-Compton scattering of cosmic microwave background photons in a single-zone jet flow. Within two-zone jet models, we find that a synchrotron origin for the jet's X-rays requires fewer additional assumptions than an inverse-Compton model, especially if velocity shear leads to efficient particle acceleration in jet flows. 
S. Jester (Fermilab and U. Southampton), D. E. Harris (SAO), H. L.   Marshall (MIT Kavli Institute), K. Meisenheimer (MPIA H...