Non-thermal and relativistic phenomena in galaxy clusters

The cosmic ray electrons which are responsible for the radio-halo and relic synchrotron emission inevitably Compton scatter the CMB (as well as other local background) photons which will then gain energy and emit at higher energy E ss 2.7 keV (E/GeV)2. Electrons with E E a few GeV produce emission in the HXR range, while electrons with E 400 MeV produce soft X-rays and UV emission. There is actually evidence for an excess of emission w.r.t. the thermal bremsstrahlung emission by the hot IC gas in about 20 nearby clusters 

Several galaxy clusters show also an emission of extreme UV (Lieu et al. 1996, Durret et al. 2002) and soft X-ray (Bonamente et al. 2002, Kaastra et al. 2002) radiation in excess w.r.t. the thermal bremsstrahlung emission. This EUV emission excess may be consistent with both ICS of CMB photons off a non-thermal electron population (e.g., Lieu et al. 1999, Bowyer 2000) with Ee = 608.5 MeV (hv/keV)1/2 149 MeV for hv 60 eV, and with thermal emission from a warm gas at ksTe V 1 keV (Bonamente et al. 2002). In the case of Coma, the simple extrapolation of the ICS spectrum which fits the HXR excess down to energies 0.25 keV does not fit the EUV excess measured in Coma because it is too steep and yields a too high flux compared to the measured flux by the EUV satellite in the 0.065 — 0.245 keV band (Ensslin Biermann 1998). Thus, under the assumption that the HXR and the EUV emission of Coma is produced by ICS of CMB photons, the minimal requirement is that a break in the electron spectrum should be present in the range 0.3 — 2.8 GeV in order to avoid an excessive EUV contribution by the ICS emission and to be consistent with the radio halo spectrum. 

This mechanism has an essential ingredient in the confinement of the CRs within clusters where they are produced (e.g., Colafrancesco Blasi 1998). 

A similar mechanism of gamma-ray emission can be also provided by the annihilation of Dark Matter particles (see Colaffancesco at this Meeting) which produce both direct gamma-ray emission and secondarily produced gamma-rays through 7r° — 7 I 7, as above. 

There is not yet, however, a definite detection of diffuse gamma-ray emission from galaxy clusters. While there is a preliminary evidence of gamma-ray emission from a dozen bright, radio-active clusters which host powerful radio galaxies and Blazars and are associated to unidentified EGRET sources (Co-lafrancesco 2002), many of the quiet, X-ray selected clusters only have upper limits for their emission at E 100 MeV. 

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