The emission processes responsible for the X-rays are discussed in detail. The radio-optical spectrum of the hot spot breaks or turns down at 10 Hz, and its X-ray spectrum is not a simple extension of the radio-optical spectrum to higher frequencies. Thermal models for the hot spot's X-ray emission are ruled out. Synchrotron self-Compton models involving scattering from the known population of electrons give the wrong spectral index for the hot spot's X-ray emission and are also excluded. A composite synchrotron plus synchrotron self-Compton model can match the X-ray observations but requires similar contributions from the two components in the Chandra band. We show that the hot spot's X-ray emission could be synchrotron self-Compton emission from a hitherto unobserved population of electrons emitting at low radio frequencies, but do not favor this model in view of the very weak magnetic field required. For the jet, inverse Compton models require a magnetic field a factor of 30 below equipartition, and ad hoc conditions to explain why the radio lobes are fainter than the jet in X-rays, but brighter in the radio.
Synchrotron radiation is the favored process for the X-ray emission. The expected synchrotron spectrum from relativistic electrons accelerated by strong shocks and subject to synchrotron radiation losses is in very good agreement with that observed for both the hot spot and jet. The possibility that the relativistic electrons result via photo-pion production by high energy protons accelerated in shocks (a `proton induced cascade') is briefly discussed.
Keywords: galaxies: active - galaxies: individual (Pictor A) - galaxies: jets - galaxies: nuclei - ISM: cosmic rays - X-rays: galaxies