Illuminating a Dark Lens : A Type Ia Supernova Magnified by the Frontier
  Fields Galaxy Cluster Abell 2744
  SN HFF14Tom is a Type Ia Supernova (SN) discovered at z = 1.3457 +- 0.0001
behind the galaxy cluster Abell 2744 (z = 0.308). In a cosmology-independent
analysis, we find that HFF14Tom is 0.77 +- 0.15 magnitudes brighter than
unlensed Type Ia SNe at similar redshift, implying a lensing magnification of
mu_obs = 2.03 +- 0.29. This observed magnification provides a rare opportunity
for a direct empirical test of galaxy cluster lens models. Here we test 17 lens
models, 13 of which were generated before the SN magnification was known,
qualifying as pure "blind tests". The models are collectively fairly accurate:
8 of the models deliver median magnifications that are consistent with the
measured mu to within 1-sigma. However, there is a subtle systematic bias: the
significant disagreements all involve models overpredicting the magnification.
We evaluate possible causes for this mild bias, and find no single physical or
methodological explanation to account for it. We do find that model accuracy
can be improved to some extent with stringent quality cuts on multiply-imaged
systems, such as requiring that a large fraction have spectroscopic redshifts.
In addition to testing model accuracies as we have done here, Type Ia SN
magnifications could also be used as inputs for future lens models of Abell
2744 and other clusters, providing valuable constraints in regions where
traditional strong- and weak-lensing information is unavailable.
