2. EUV EMISSION

The EUV excess in the 0.07 to 0.14 keV range was first detected by the Extreme Ultraviolet Explorer from Coma (Lieu et al. 1996) and some other clusters. There are claimed detections of similar excess emissions in the 0.1 to 0.4 keV band by Rosat, BeppoSAX and XMM-Newton. The observational problems related to the EUV and soft excesses are discussed by Durret et al. 2008 - Chapter 4, this volume. Initially, these excesses were attributed to thermal emission by a cooler (kT ~ 2 keV) component, but there are several theoretical arguments against this possibility, most notable is that the expected line emission is not observed. The alternative model is the IC scattering by CMB photons, which, in principle, can be easily fitted over the small range of observations. However, this will require a population of lower energy ( ~ 10³) electrons, indicating that the power law distribution required for production of radio radiation must be extended to lower energies with a power law index p ~ 3. This of course will mean an order of magnitude more energy in electrons and it makes equipartition less likely (see also the discussion at the end of Sect. 3.2.2)).

In summary, some of the observations of the EUV emission are widely questioned and their theoretical modelling is quite problematic.