  The steep mass profile of A1689 derived from recent detailed lensing
observations is not readily reconciled with the low concentration halos
predicted by the standard CDM model. However, halo triaxiality may act to bias
the profile constraints derived assuming a spherically symmetric mass
distribution, since lensing relates only to the projected mass distribution. A
degree of halo triaxiality is inherent to the CDM structure formation, arising
from the collision-less nature of the dark matter. Here we compare the
CDM-based model predictions of triaxial halo with the precise lensing
measurements of A1689 based on the ACS/HST and Subaru data, over a wide range
of 10kpc<r<2Mpc. The model lensing profiles cover the intrinsic spread of halo
mass and shape, and are projected over all inclinations when comparing with the
data. We show that the model parameters are only weakly constrained and
strongly degenerate mainly because of the lack of information along the line of
sight. In particular, the limits on the concentration parameter become less
restrictive with increasing triaxiality. Yet, by comparing the obtained
constraints with expected probability distributions for the axis ratio and
concentration parameters computed from numerical simulations, we find that ~6%
of cluster-size halos in the CDM model can match the A1689 lensing observations
at the 2-sigma level, corresponding to cases where the major-axis of the halo
is aligned with the line of sight. Thus halo triaxiality could reduce the
apparent discrepancy between theory and observation. This hypothesis needs to
be further explored by a statistical lensing study for other clusters as well
as by complementary three-dimensional information derived using X-ray,
kinematics, and SZ effect observations.
