Shock waves in the ICM are the most prominent features emerging from mergers. Due to relative velocities of the colliding subclusters of up to ~ 3000 km/s, shocks of Mach numbers up to about 3 are produced. These are relatively mild shocks.

When a dense subcluster falls into a cluster a shock is observed before the core passage, and it manifests itself as a bow shock visible in front of the infalling subcluster (Roettiger et al. 1997). The strongest shock waves are produced after the collision of subclusters, and they propagate outwards along the original collision axis (Schindler & Müller 1993; Roettiger et al. 1999a; see Figs. 8.1e and 8.2e). The shocks are visible as steep gradients in the gas density and in the gas temperature. In general, the shock structure is found to be more filamentary at early epochs and quasi-spherical at low redshifts (Fig. 8.3; Quilis et al. 1998).

Observationally, the shocks are best visible in X-ray temperature maps, because they show up as steps in these maps (Schindler & Müller 1993, see Fig 8.2e). They are visible also in X-ray surface brightness images, even if less prominently, because other effects like for example the presence of substructure, can cause irregularities in the images. Therefore it is ideal to measure and compare both, either the X-ray surface brightness image and the temperature map.

For the temperature maps, spatially resolved X-ray spectroscopy is necessary which can be performed now with high accuracy thanks to the new X-ray observatories XMM and CHANDRA. A number of clusters shocks and complicated temperature distributions in the intra-cluster gas have already been found; see for instance the Coma cluster (Arnaud et al. 2001), A665 and A2163 (Markevitch & Vikhlinin2001), and A2142 (Markevitch et al. 2000).

These shocks are not only the major source of heating for the intra-cluster gas, but they are also particularly important for particle (re-) acceleration models. Relativistic particles, which were originally emitted by active galaxies, age quickly below the detection threshold of radio telescopes. Later these particles could be (re-)accelerated to relativistic energies in the shocks.

These relativistic particles are probably responsible for the non-therma emission. Their interaction with the cluster magnetic field shows up as synchrotron emission of diffuse radio sources like radio halos and/or relics (see review by Giovannini & Feretti in this book).

The shocks heat primarily the thermal ions. This has been shown in simulations which treat ions and electrons separately (see Fig. 8.4;

Figure 8.3. Shocked cells in cubes of 20 Mpc (comoving) at different redshifts. At first the shock structure is more filamentary, later it is quasi-spherical (from Quilis et al. 1998).

Gas temperatures 8x10

Figure 8.4. Temperature of the ions (solid line) and of the electron (dashed line) versus distance from the cluster centre. At the positions of the shocks (left: 3 Mpc, right: 2 Mpc from the cluster centre) the ions have higher temperatures than the electrons (from Chieze et al. 1998).

Chieze et al. 1998; Takizawa 1999). Only later is the energy transferred also to the thermal electrons.

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