Non-parametric mass reconstruction of A1689 from strong lensing data with the Strong Lensing Analysis Package

We present the mass distribution in the central area of the cluster A1689 by fitting over 100 multiply lensed images with the non-parametric Strong Lensing Analysis Package. The surface mass distribution is obtained in a robust way, finding a total mass of 0.25 x 10(15) h(-1) M circle dot within a 70-arcsec circle radius from the central peak. Our reconstructed density profile fits well a Navarro-Frenk-White (NFW) profile with small perturbations due to substructure, and is compatible with the more model-dependent analysis of Broadhurst et al. based on the same data. Our estimated mass does not rely on any prior information about the distribution of dark matter in the cluster. The peak of the mass distribution falls very close to the central dominant (cD) galaxy and there is substructure near the centre suggesting that the cluster is not fully relaxed. We also examine the effect on the recovered mass when we include the uncertainties in the redshift of the sources and in the original shape of the sources. Using simulations designed to mimic the data, we identify some biases in our reconstructed mass distribution. We find that the recovered mass is biased toward lower masses beyond 1 arcmin (150 kpc) from the cD galaxy and that in the very centre we may be affected by degeneracy problems. On the other hand, we confirm that the reconstructed mass between 25 and 70 arcsec is a robust, unbiased estimate of the true mass distribution and is compatible with an NFW profile.