X‐Ray and Optical Emission from Radio Hot Spots of Powerful Quasars

Abstract

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 ~2 × 107. 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 δ = 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.