Next Contents Previous


Diffuse radio sources are not the only indication of non-thermal activity in the ICM. The prospects for the X-ray detection of inverse-Compton emission originating from radio emitting electrons and photons of the microwave background were presented about 30 years ago [108, 109]. Significant progress in the search of non-thermal emission in the hard X-ray band (> 20 keV, HXR) has been recently made owing to the improved sensitivity and wide spectral capabilities of the BeppoSAX and the Rossi X-ray Timing Explorer (RXTE) satellites (see the review by Fusco-Femiano et al. [110]).

Evidence for the presence of HXR radiation in excess to the thermal emission has been obtained in the spectrum of Coma [111, 112, 113, 114]. In Fig. 5, we report the combined hard X-ray spectrum of the Coma cluster, obtained using BeppoSAX data of two independent observations of 90 ksec and 300 ksec [114]. The non-thermal excess with respect to the thermal emission is at a confidence level of ~ 4.8sigma and has a flux of (1.5 ± 0.5) × 10-11 erg cm-2 s-1 in the 20-80 keV energy band (assuming a photon index GammaX = 2.0). In the framework of the IC model (see Sec. 3.4) the combination of the radio and non-thermal X-ray fluxes allows an estimate of the volume-averaged intracluster magnetic field of ~ 0.2 µG.

Figure 5

Figure 5. Coma cluster -- PDS data. The continuous line represents the best fit with a thermal component at the average cluster gas temperature of 8.1 keV. The errors bars are quoted at the 1sigma level 114.

In addition to Coma, HXR non-thermal emission has been detected in A2256 [115, 116]. The flux in the 20 - 80 keV energy range is ~ 9 × 10-12 erg cm-2 s-1. A magnetic field of ~ 0.05 µG is derived for the northern cluster region, where the radio relic is located, while a higher field value, ~ 0.5 µG, could be present at the cluster center, in the region of the radio halo. A HXR detection at low confidence level is obtained in A754 [117, 118], where however a radio galaxy with BL Lac characteristics could be responsible for the emission.

The detection in A2199, which is a cooling core cluster with no extended diffuse radio emission, is controversial [110, 119]. A marginal detection has been obtained in A119 [110], a merging cluster without a radio halo, but the presence of several point sources in the field of view makes the IC interpretation unlikely.

For the clusters A3667 [120] and A2163 [38] only upper limits to the non-thermal X-ray emission have been derived. A possible detection in A2319 with RXTE [121] leads to a magnetic field of 0.1 - 0.3 µG. Localized IC emission associated with the radio relic and with merger shocks in A85 has been claimed from ROSAT observations [122]. The derived magnetic field is ~ 1 µG.

The value of the magnetic field derived in the Coma cluster by the IC HRX emission is quite consistent with that obtained by the radio halo Coma C under equipartition conditions (Sec. 4.1), but it is much lower than that derived from RM measurements (see next section). Therefore, alternative interpretations to the IC model for the non-thermal radiation detected in the Coma cluster have been proposed. A suggested mechanism is the non-thermal bremsstrahlung from supra-thermal electrons formed through the current acceleration of the thermal gas [123, 124, 125, 126]. However, Petrosian [127] pointed out that due to the low efficiency of the bremsstrahlung mechanism, these models would require an unrealistically high energy input.

We will show in Sec. 9 that the disagreement between IC and RM magnetic field measurements can be at least partially alleviated. Future studies of non-thermal X-ray emission in clusters will be possible with the ASTRO-E2 satellite.

Next Contents Previous