Based on a 21.5 ks Chandra observation of A2556 , we identify an edge on the surface brightness profile ( SBP ) at about 160 h _ { 71 } ^ { -1 } kpc northeast of the cluster center , and it corresponds to a shock front whose Mach number \mathcal { M } is calculated to be 1.25 _ { -0.03 } ^ { +0.02 } .
No prominent substructure , such as sub-cluster , is found in either optical or X-ray band that can be associated with the edge , suggesting that the conventional super-sonic motion mechanism may not work in this case .
As an alternative solution , we propose that the nonlinear steepening of acoustic wave , which is induced by the turbulence of the ICM at the core of the cluster , can be used to explain the origin of the shock front .
Although nonlinear steepening weak shock is expected to occur frequently in clusters , why it is rarely observed still remains a question that requires further investigation , including both deeper X-ray observation and extensive theoretical studies .