1. INTRODUCTION

The observed signatures of the non-thermal (NT) activity in the intra-cluster medium (ICM) were described in details by Rephaeli et al. 2008 - Chapter 5, this volume and Ferrari et al. 2008 - Chapter 6, this volume. Here we give a brief summary. The first and least controversial radiative signature comes from radio observations. Synchrotron emission by a population of relativistic electrons is the only possible model for the production of this radiation. In the case of the Coma cluster, the radio spectrum may be represented by a broken power law (Rephaeli 1979), or a power law with a rapid steepening (Thierbach et al. 2003), or with an exponential cutoff (Schlickeiser et al. 1987), implying the presence of electrons with similar spectra. Unfortunately, from radio observations alone one cannot determine the energy of the electrons or the strength of the magnetic field. Additional observations or assumptions are required. Equipartition or minimum total (relativistic particles plus fields) energy arguments imply a population of relativistic electrons with Lorentz factor ~ 10⁴ and magnetic field strength of B ~ µG, in rough agreement with the Faraday rotation measurements (e.g. Kim et al. 1990). Rephaeli (1979) and Schlickeiser et al. (1987) also pointed out that the electrons responsible for the radio emission, should also produce a spectrum of hard X-ray (HXR) photons (similar to that observed in the radio band), via inverse Compton (IC) scattering of the Cosmic Microwave Background (CMB) photons. This emission is estimated to be the dominant emission component around 50 keV. Detection of HXR radiation could break the degeneracy and allow determination of the magnetic field and the energy of the radiating electrons. In fact, because the energy density of the CMB radiation (temperature T₀) u_CMB = 4 × 10^-13(T₀ / 2.8 K)⁴ erg cm^-3 is larger than the magnetic energy density u_B = 3 × 10^-14(B / µG)² erg cm^-3, one expects a higher flux of HXR than radio radiation.

As already described in the above mentioned papers by Rephaeli et al. and Ferrari et al., recently there has been growing evidence for this and other signatures of the NT activity. Excess HXR and extreme ultraviolet (EUV) radiation are observed at the high and low ends of the usual soft X-ray (SXR) thermal Bremsstrahlung (TB) radiation. Fig. 1 shows all the flux F () (or equivalently the energy density u() = 4 F () / c) of the above mentioned and other radiation for the Coma cluster. However, for the excess radiation not only the exact mechanisms are controversial but even their NT nature is questioned. The observed spectra of the excess radiation often can be fit by thermal spectra with higher and lower temperatures than that needed for the SXR observations with almost the same confidence as with a NT power law. The most natural NT process for these excesses (specially for HXRs) is the IC scattering of the CMB photons. However, the relatively high observed HXR fluxes require a large number of relativistic electrons, and consequently a relatively low magnetic field for a given observed radio flux. For Coma, this requires the (volume averaged) magnetic field to be B ~ 0.1-0.3 µG, while equipartition gives B ~ 0.4 µG and Faraday rotation measurements give the (average line-of-sight) field of B ~ 3 µG (Giovannini et al. 1993, Kim et al. 1990, Clarke et al. 2001, Clarke 2003). In general the Faraday rotation measurements of most clusters give B > 1 µG; see e.g. Govoni et al. (2003). Because of this apparent difficulty, various authors (see, e.g. Enßlin et al. 1999, Blasi 2000) suggested that the HXR radiation is due to non-thermal Bremsstrahlung by a second population of NT electrons with a power law distribution in the 10 to 100 keV range. In what follows we examine the merits and shortcomings of the mechanisms proposed to interpret these observations. We first consider the EUV observations briefly and then address the thermal and NT (IC and non-thermal Bremsstrahlung) radiation model for the HXR observations.

Figure 1. The flux of all observed electromagnetic radiation for the Coma cluster including cosmic microwave background (CMB), cosmic background light (CBL) and static magnetic field (obtained from their energy density u() as F()= (c / 4) × u()). The spectra shown for the EUV and HXR range are schematic and the upper limit in gamma ray range is from EGRET (Sreekumar et al. 1996). From Petrosian (2003).