Spitzer Observations of Hotspots in Radio Lobes
  We have carried out a systematic search with Spitzer Warm Mission and
archival data for infrared emission from the hotspots in radio lobes that have
been described by Hardcastle et al. (2004). These hotspots have been detected
with both radio and X-ray observations, but an observation at an intermediate
frequency in the infrared can be critical to distinguish between competing
models for particle acceleration and radiation processes in these objects.
Between the archival and warm mission data, we report detections of 18
hotspots; the archival data generally include detections at all four IRAC
bands, the Warm Mission data only at 3.6 microns.
  Using a theoretical formalism adopted from Godfrey et al. (2009), we fit both
archival and warm mission spectral energy distributions [SEDs] - including
radio, X-ray, and optical data from Hardcastle as well as the Spitzer data -
with a synchrotron self-Compton [SSC] model, in which the X-rays are produced
by Compton scattering of the radio frequency photons by the energetic electrons
which radiate them. With one exception, a SSC model requires that the magnetic
field be less or much less than the equipartition value which minimizes total
energy and has comparable amounts of energy in the magnetic field and in the
energetic particles. This conclusion agrees with those of comparable recent
studies of hotspots, and with the analysis presented by Hardcastle et al.
(2004). We also show that the infrared data rule out the simplest synchrotron
only models for the SEDs. We briefly discuss the implications of these results
and of alternate interpretations of the data.
