2007ApJ...662..213S

Here we present Spitzer Space Telescope imaging of Cyg A with the Infrared Array Camera at 4.5 and 8.0 µm, resulting in the detection of the high-energy tails in the synchrotron spectra for all four hotspots of this radio galaxy. When combined with the other data collected from the literature, our observations allow for detailed modeling of the broadband 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 intensityB~170 µG in hotspot A, and B~270 µG 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 ∼100 GeV, and that the spectral break corresponds almost exactly to the proton rest energy of ∼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.