X-ray and optical emission from radio hot spots of powerful quasars

In a survey with Chandra and HST of a sample of 17 radio sources with bright radio jets ( 16 powerful FR II and one nearby FR I), we detected X- ray and optical emission from a number of radio hot spots and lobes. Six hot spots on the near sides of powerful FR II galaxies ( as determined from the jet asymmetry) were detected at X- rays, while none were detected on the far side, suggesting that high- energy emission from hot spots is anisotropic. In the nearby FR I galaxy 0836+ 299 ( the only FR I in our sample) both hot spots are detected in X- rays, in agreement with the symmetric radio morphology. In the latter case the spectral energy distributions ( SEDs) of both hot spots can be modeled from radio to X- rays with synchrotron emission from a single power- law energy distribution of electrons with Lorentz factors up to similar to 2; 10(7). For the six hot spots of powerful FR II galaxies the X- ray flux lies above the extrapolation from the radio- to- optical continuum. Modeling the SEDs with a one- zone synchrotron self- Compton model, we find that equipartition is strongly violated, with the particle energy density dominating over the magnetic field one by 1 - 2 orders of magnitude. We discuss alternatives to this simple model, concluding that a viable alternative is that the X- ray emission is produced in the still- relativistic ( Doppler factor delta = 3- 6) terminal part of the jet by inverse Compton ( IC) scattering on the CMB or synchrotron photons emitted by plasma flowing with a small velocity. X- ray emission from some of the lobes is detected on the side opposite to the jet, suggesting the possible relevance of back- scattered central radiation in providing seed photons for the IC process.