The origins of X-ray emission from the hot spots of FR II radio sources

We use new and archival Chandra data to investigate the X-ray emission from a large sample of compact hot spots of FR II radio galaxies and quasars from the 3C catalog. We find that only the most luminous hot spots tend to be in good agreement with the predictions of a synchrotron self-Compton model with equipartition magnetic fields. At low hot spot luminosities inverse Compton predictions are routinely exceeded by several orders of magnitude, but this is never seen in more luminous hot spots. We argue that an additional synchrotron component of the X-ray emission is present in low-luminosity hot spots and that the hot spot luminosity controls the ability of a given hot spot to produce synchrotron X-rays, probably by determining the high-energy cutoff of the electron energy spectrum. It remains plausible that all hot spots are close to the equipartition condition.