Chandra Observation of a Weak Shock in the Galaxy Cluster A2556

TL;DR

This paper reports the detection of a weak shock front in galaxy cluster A2556 using Chandra X-ray observations, suggesting nonlinear steepening of acoustic waves as a possible origin, which is rarely observed and warrants further study.

Contribution

It introduces the first evidence of a weak shock in A2556 and proposes nonlinear acoustic wave steepening as an alternative shock formation mechanism.

Findings

Detected a shock front with Mach number ~1.25 in A2556.

No substructure found, ruling out sub-cluster motion.

Highlights the rarity of observing such weak shocks in clusters.

Abstract

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.