Omnidirectional Inverse Compton and Synchrotron Radiation from Cosmic Distributions of Fast Electrons and Thermal Photons

This paper is an attempt to interpret the observed diffuse component of cosmic X-rays and gamma rays as inverse Comton radiation from relativistic electrons in tenuous regions of space. The inverse Compton effect (prouction of a high-energy photon by the collision of a fast electron with a low-energy photon) is discussed kinematically in the Thomson approximation, and useful formulae are obtained. For the special case of thermal photon and power-law electron distributions, explicit brightness formulae for astrophysical use are derived; a function approximation for scattering by a single electron is employed. Similar brightness formulae are derived for synchrotron emission; the limitations of the derivation are examined and the formal analogy between the two processes is emphasized. A qualitative comparison is made between expected synchrotron and Compton spectra of the galactic halo. The electron spectrum of the halo is discussed in some detail, and its probable form is derived on the basis of equilibrium against energy losses in a uniform diffusive trapping region. The diffusive escape time is assumed to be 108 years. Inverse Compton losses are taken into account; starlight energy density in the halo is estimated to be 10- eV . The resulting galactic electron spectrum is normalized to radio observations, and the expected inverse Compton flux due to interaction of starlight with the same electron spectrum is calculated. The process is repeated with the additional assumption that cosmic black-body radiation at 3 K is present. These two curves are compared with collected X- and gamma-ray data; the higher of the two (the 3 K curve) lies 2 orders of magnitude too low. It is concluded that halo electrons are insufficient to roduce the observed radiation. The shape of the spectrum they produce is, however, compatible with t at observed. Emission by halos of external galaxies is discussed briefly but is likewise insufficient. Other metagalactic models are considered, in which electrons are present not only in galaxies but also in tenuous regions of low field and possibly spread throughout space. Equilibrium spectra for such electrons are derived and then normalized to the observed X-ray spectrum. If the 3 K radiation is present, the required electron spectrum has differential exponent -3.4 and lies about a factor of 10 below the halo spectrum in the region 1-10 0eV. Possible sources of injection for such a metagalactic flux are discussed; secondary production by a universal cosmic-ray flux and escape of electrons from strong radio galaxies are viable source mechanisms. Regardless of source theories, the metagalactic electron flux in 0.5-10 0eV has some empirical footing in the X-ray observations.