2002A&A...383..423E

Most of the Universe's populations of low energy cosmic ray electrons in the energy range of 1-100MeV still manage to elude detection by our instruments, since their synchrotron emission is at too low frequencies. We investigate a mechanism which can lead to observable emission of such electron populations: synchrotron-self Comptonization (SSC). The inverse Compton (IC) scattering can shift otherwise unobservable low-frequency 10kHz-10MHz photons into observable radio, infrared (IR) or even more energetic wave-bands. The resulting emission should be polarized. We also consider IC scattering of the cosmic microwave background (CMB) and the cosmic radio background (CRB). Electron spectral aging due to synchrotron, IC and adiabatic losses or gains influences the resulting spectrum. The predicted radiation spectrum is a strong function of the history of the source, of the low energy spectrum of relativistic electrons, and of redshift. It has typically two maxima, and a decrement in between at CMB frequencies. Detection will provide a sensitive probe of the environment of radio galaxies. We demonstrate that the fossil remnants of powerful radio galaxies are promising detection candidates, especially when they are embedded in a dense intra-cluster medium (ICM). GHz peaked sources (GPS) have very low SSC luminosities, which may, however, extend into the X-ray or even the gamma ray regime. Clusters of galaxies with relativistic electron populations may be another detectable sources. Fossil radio plasma released by our own Galaxy could be revealed by its large angular scale SSC flux. We discuss the expected detectability of these sources with new and upcoming instruments such as LOFAR, GMRT, EVLA, ATA, ALMA, PLANCK, and HERSCHEL.