On the Nonthermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies

Abstract

Some clusters of galaxies, in addition to thermal bremsstrahlung (TB), emit detectable diffuse radiation from the intracluster medium (ICM) at radio, EUV, and hard X-ray (HXR) ranges. The radio radiation must be due to synchrotron by relativistic electrons, and the inverse Compton scattering by the cosmic microwave background radiation of the same electrons is the most natural source for the HXR and perhaps the EUV emissions. However, simple estimates give a weaker magnetic field than that suggested by Faraday rotation measurements. Consequently, nonthermal bremsstrahlung (NTB) and TB have also been suggested as sources of these emissions. We show that NTB cannot be the source of the HXRs (except for a short period) and that the difficulty with the low magnetic field in the IC model is alleviated if the effects of observational selection bias, nonisotropic pitch angle distribution, and spectral breaks in the energy distribution of the relativistic electrons are taken into account. From these considerations and the strength of the EUV emission, we derive a spectrum for the radiating electrons and discuss possible acceleration scenarios for its production. We show that continuous and in situ acceleration in the ICM of the background thermal electrons is difficult and requires unreasonably high energy input. Similarly, acceleration of injected relativistic electrons, say, by galaxies, seems unreasonable because it will give rise to a much flatter spectrum of electrons than required, unless a large fraction of energy input is carried away by electrons escaping the ICM, in which case one obtains EUV and HXR emissions extending well beyond the boundaries of the diffuse radio source. A continuous emission by a cooling spectrum resulting from interaction with ICM of electrons accelerated elsewhere also suffers from similar shortcomings. The most likely scenario appears to be an episodic injection acceleration model, whereby one obtains a time-dependent spectrum that for certain phases of its evolution satisfies all the requirements.