Pairs of Giant Shock Waves (N-Waves) in Merging Galaxy Clusters
  When a subcluster merges with a larger galaxy cluster, a bow shock is driven
ahead of the subcluster. At a later merger stage, this bow shock separates from
the subcluster, becoming a "runaway" shock that propagates down the steep
density gradient through the cluster outskirts and approximately maintains its
strength and the Mach number. Such shocks are plausible candidates for
producing radio relics in the periphery of clusters. We argue that, during the
same merger stage, a secondary shock is formed much closer to the main cluster
center. A close analog of this structure is known in the usual hydrodynamics as
N-waves, where the trailing part of the "N" is the result of the non-linear
evolution of a shock. In merging clusters, spherical geometry and
stratification could further promote its development. Both the primary and the
secondary shocks are the natural outcome of a single merger event and often
both components of the pair should be present. However, in the radio band, the
leading shock could be more prominent, while the trailing shock might
conversely be more easily seen in X-rays. The latter argument implies that for
some of the (trailing) shocks found in X-ray data, it might be difficult to
identify their "partner" leading shocks or the merging subclusters, which are
farther away from the cluster center. We argue that the Coma cluster and A2744
could be two examples in a post-merger state with such well-separated shock
pairs.
