  Deep Chandra exposures of the hot galaxy cluster 1E0657-56 have revealed that
the cluster is observed shortly after the first core-passage of a massive
infalling subcluster, which is preceded by a prominent bow shock with Mach
number M~3. The inferred shock velocity of ~4700 km/s has been commonly
interpreted as the velocity of the `bullet' subcluster itself. This velocity is
unexpectedly high in the LCDM cosmology, which may require non-trivial
modifications in the dark sector to be accommodated if taken at face value.
Here we present explicit hydrodynamical toy models of galaxy cluster mergers
which very well reproduce the observed dynamical state of 1E0657-56 and the
mass models inferred from gravitational lensing observations. However, despite
a shock speed of 4500 km/s, the subcluster's mass centroid is moving only with
2600 km/s in the rest frame of the system. The difference arises in part due to
a gravitationally induced inflow velocity of the gas ahead of the shock, which
amounts to ~1100 km/s for our assumed 10:1 mass ratio of the merger. A second
effect is that the shock front moves faster than the subcluster itself. A
generic LCDM collision model, where a bullet subcluster with concentration
c=7.2 merges with a parent cluster with concentration c=3 on a zero-energy
orbit, reproduces all the main observational features seen in 1E0657-56 with
good accuracy, suggesting that 1E0657-56 is well in line with expectations from
standard cosmological models. In theories with an additional 5th-force in the
dark sector, the subcluster can be accelerated beyond the velocity reached in
LCDM, and the spatial offset between the X-ray peak and the mass centroid of
the subcluster can be significantly enlarged. (abridged)
