DISCREPANT MASS ESTIMATES IN THE CLUSTER OF GALAXIES ABELL 1689

We present a new mass estimate of a well studied gravitational lensing cluster, Abell 1689, from deep Chandra observations with a total exposure of 200 ks. Within r = 200 h(-1) kpc, the X-ray mass estimate is systematically lower than that of lensing by 30%-50%. At r > 200 h(-1) kpc, the mass density profiles from X-ray and weak lensing methods give consistent results. The most recent weak lensing work suggests a steeper profile than what is found from the X- ray analysis, while still in agreement with the mass at large radii. Fitting the total mass profile to a Navarro-Frenk-White model, we find M(200) = (1.16(-0.27)(+ 0.45)) x 10(15) h(-1) M(circle dot) with a concentration, c(200) = 5.3(-1.2)(+1.3), using nonparametric mass modeling. With parametric profile modeling, we find M(200) = (0.94(-0.06)(+0.11)) x 10(15) h(-1) M(circle dot) and c(200) = 6.6(-0.4)(+0.4). This is much lower compared to masses deduced from the combined strong and weak lensing analysis. Previous studies have suggested that cooler small-scale structures can bias X- ray temperature measurements or that the northern part of the cluster is disturbed. We find these scenarios unlikely to resolve the central mass discrepancy since the former requires 70%-90% of the space to be occupied by these cool structures, and excluding the northern substructure does not significantly affect the total mass profiles. A more plausible explanation is a projection effect. Assuming that the gas temperature and density profiles have a prolate symmetry, we can bring the X- ray mass estimate into a closer agreement with that of lensing. We also find that the previously reported high hard-band to broadband temperature ratio in A1689, and many other clusters observed with Chandra, may be resulting from the instrumental absorption that decreases 10%-15% of the effective area at similar to 1.75 keV. Caution must be taken when analyzing multiple spectral components under this calibration uncertainty.