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The high sensitivity and resolution of the Chandra satellite has allowed the detection in the clusters A2142 and A3667 of sharp discontinuities in the X-ray surface brightness, the so called cold fronts [151, 152, 153]. Ettori and Fabian [154] pointed out that the observed temperature jumps in A2142 require that thermal conduction across cold fronts must be suppressed by a factor of 100 or more, compared to the classical Spitzer value [155]. Actually, a tangled magnetic field has been found [156] to reduce the thermal conductivity from the Spitzer value by a factor of order 102 - 103.

Similar features have now been detected in several other clusters [65, 157, 158]. These structures are apparently contact discontinuities between the gas which was in the cool core of one of the merging sub-clusters and the surrounding intracluster gas. They are not shocks because the density increase across the front is accompanied by a temperature decrease such that there is no dramatic change in the pressure and entropy.

In the cluster A3667 [152, 153] (see Fig. 7), the temperature discontinuity in the cold front occurs over a scale of 3.5" (appeq 4 kpc). Vikhlinin et al. [153] suggested that in order to reproduce such a sharp feature, magnetic fields are required to suppress both thermal conduction and Kelvin-Helmholtz instabilities along the contact discontinuity. They found that the front sharpness and its gradual smearing at large angles are most likely explained by the existence of a layer with a appeq 10 µG magnetic field parallel to the front. The magnetic filed in the layer is probably amplified by the stretching of the field lines.

Figure 7

Figure 7. Chandra X-ray image (left) and temperature map (right) of A3667 [152, 153].

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