Our understanding of the physics of kpc-scale quasar jets had seemed to converge to a paradigm in which these jets are as highly relativistic on the kpc scale as they are on sub-pc scales close to the central black hole. Retaining bulk Lorentz factors (Γ) on the order of 10–20 at these distances implies a jet power comparable to or higher than their Eddington luminosity. We recently started challenging this paradigm, which was put in place to explain the surprisingly bright X-ray emission of the knots of many quasar jets as inverse Compton scattering off the cosmic microwave background (IC/CMB). We have shown that the knot X-ray emission of the archetypical jets 3C 273 and PKS 0637-752 is not due to IC/CMB. With IC/CMB disfavored, an alternative interpretation for the X-rays is synchrotron radiation from a second population of electrons accelerated in situ up to ∼100 TeV. These results are the first step towards resolving the long-standing issue of the nature of the X-ray emission in powerful quasar jets. Comprehensive observational and theoretical work on essentially all X-ray-detected large-scale quasar jets to test the IC/CMB model over a much larger population needs to be done to examine the implications of slower jets that are extremely efficient accelerators. A fascinating case can be made that—contrary to popular belief—the total radiative power of the large-scale jet of these sources is comparable to that of the quasar core. Even more so, the angle-integrated TeV output of these (previously thought TeV-quiet) quasar jets likely makes them the dominant class among active galactic nuclei (AGN), exceeding the TeV production of so-called TeV blazars.
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