2009ApJ...701.1283P

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 M₂₀₀= (1.16^+0.45_–0.27)x10¹⁵ h^–1.M_☉ with a concentration, c₂₀₀= 5.3^+1.3_–1.2, using nonparametric mass modeling. With parametric profile modeling, we find M₂₀₀= (0.94^+0.11_–0.06)x10¹⁵ h^–1. M_☉ and c₂₀₀= 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 ∼1.75 keV. Caution must be taken when analyzing multiple spectral components under this calibration uncertainty.