Spitzer Observations of Hotspots in Radio Lobes

TL;DR

This study uses Spitzer infrared observations combined with radio and X-ray data to analyze hotspots in radio lobes, supporting synchrotron self-Compton models with magnetic fields below equipartition, and ruling out simple synchrotron models.

Contribution

First systematic infrared survey of radio lobe hotspots, providing critical data to test particle acceleration models and confirming SSC models with sub-equipartition magnetic fields.

Findings

Infrared data detect 18 hotspots, constraining their spectral energy distributions.

SSC models fit the data well, requiring magnetic fields below equipartition.

Simple synchrotron models are ruled out by infrared observations.

Abstract

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.