2002ApJ...576..152X

We present a new estimate of the projected X-ray mass of A1689 observed with Chandra X-Ray Observatory, in an attempt to clarify the issue of whether or not there exists a discrepancy between X-ray and gravitational lensing mass estimates claimed in previous investigations based on Einstein, ROSAT, and ASCA observations. Particular attention is paid to examining whether there is an offset between the X-ray centroid and the central dominant cD galaxy, which may be an indicator of the presence of local dynamical activities of intracluster gas in the central core and therefore explain the discrepancy between X-ray and lensing mass estimates, if any. The unprecedentedly high spatial resolution achieved by Chandra allows us to precisely localize the X-ray centroid of A1689, which appears to coincide perfectly with the central cD galaxy. This fact, along with the symmetry and regularity of the X-ray surface brightness and temperature distributions, suggests that A1689 is a fully relaxed cluster. We thus employ the hydrostatic equilibrium hypothesis to determine the projected mass profile of A1689 and compare it with the results obtained by different lensing techniques available in the literature. Our analysis confirms the existence of a discrepancy of a factor of ∼2 between X-ray and lensing mass estimates in the central region of r~0.2 Mpc, although the two methods yield essentially consistent result on large radii. If the perfect coincidence between the X-ray center and the cD galaxy of A1689 detected by Chandra observation is not a projection effect, the central mass discrepancy between X-ray and lensing measurements may pose a challenge to our conventional understanding of the dynamical evolution of the intracluster gas in the central regions of clusters.