Chandra observation of RX J1720.1+2638: A nearly relaxed cluster with a fast-moving core?

We have analyzed the Chandra observation of the distant (z = 0.164) galaxy cluster RX J1720.1+2638, in which we find sharp features in the X-ray surfa ce brightness on opposite sides of the X-ray peak: an edge at about 250h(50 )(-1) kpc to the southeast and a plateau at about 130h(50)(-1) kpc to the n orthwest. The surface brightness edge and the plateau can be modeled as a g as density discontinuity (jump) and a slope change (break). The temperature profiles suggest that the jump and the break are the boundaries of a centr al, group-size (d approximate to 380h(50)(-1) kpc), dense, cold (T approxim ate to 4 keV) gas cloud, embedded in a diffuse hot (T approximate to 10 keV ) intracluster medium. The density jump and the temperature change across t he discontinuity are similar to the "cold fronts" discovered by Chandra in A2142 and A3667 and suggest subsonic motion of this central gas cloud with respect to the cluster itself. The most natural explanation is that we are observing a merger in the very last stage before the cluster becomes fully relaxed. However, the data are also consistent with an alternative scenario in which RX J1720.1+2638 is the result of the collapse of two co-located d ensity perturbations, the first a group-scale perturbation collapse followe d by a second cluster-scale perturbation collapse that surrounded, but did not destroy, the first one. We also show that, because of the core motion, the total mass inside the cluster core, derived under the assumption of hyd rostatic equilibrium, may underestimate the true cluster mass. If widesprea d, such motion may partially explain the discrepancy between X-ray and the strong-lensing mass determinations found in some clusters.