Chandra observation of a cold front in Abell 2554
  We present the evidence for the existence of substructure in the cold front
cluster A2554 based on a 20.14 ks Chandra observation. Using centroid shift and
X-ray brightness concentration parameters, we confirm that A2554 is a
dynamically disturbed system. We detect two dominant structures; a main cluster
at z = 0.1108 and a foreground northern substructure at z = 0.1082. The
analysis reveals an X-ray surface brightness edge at r \simeq 60 kpc from the
cluster core. The thermodynamical profiles across the edge are ruling out the
shock scenario. The temperature jump (from \sim 6 keV to \sim 10 keV), and
pressure equilibrium (P0/P1 = 1.01 \pm 0.23) across the edge, are consistent
with the definition of a cold front with a Mach number M=0.94^{+0.13}_{-0.17}
also observed a weak bow-shock at \sim 100 kpc in front of the cold cloud,
corresponding an upper limit to the Mach number M \sim 1.1. If the northern
substructure was not related to the cold front, we conclude that the transonic
motion of the cloud is caused by a merger, which was weak or occurred long ago.
