Here we present Spitzer Space Telescope imaging of Cyg A with the Infrared Array Camera at 4.5 \mu m and 8.0 \mu m , resulting in the detection of the high-energy tails or cut-offs in the synchrotron spectra for all four hotspots of this archetype radio galaxy .
When combined with the other data collected ( and re-analyzed ) from the literature , our observations allow for detailed modeling of the broad-band ( radio-to-X-ray ) emission for the brightest hotspots A and D. We confirm that the X-ray flux detected previously from these features is consistent with the synchrotron self-Compton radiation for the magnetic field intensity B \approx 170 \mu G in hotspot A , and B \approx 270 \mu G in hotspot D. We also find that the energy density of the emitting electrons is most likely larger by a factor of a few than the energy density of the hotspots ’ magnetic field .
We construct energy spectra of the radiating ultrarelativistic electrons .
We find that for both hotspots A and D these spectra are consistent with a broken power-law extending from at least 100 MeV up to \sim 100 GeV , and that the spectral break corresponds almost exactly to the proton rest energy of \sim 1 GeV .
We argue that the shape of the electron continuum most likely reflects two different regimes of the electron acceleration process taking place at mildly relativistic shocks , rather than resulting from radiative cooling and/or absorption effects .
In this picture the protons ’ inertia defines the critical energy for the hotspot electrons above which Fermi-type acceleration processes may play a major role , but below which the operating acceleration mechanism has to be of a different type .
At energies \gtrsim 100 GeV , the electron spectra cut-off/steepen again , most likely as a result of spectral aging due to radiative loss effects .
We discuss several implications of the presented analysis for the physics of extragalactic jets .