Line-of-sight Elongation and Hydrostatic Mass Bias of the Frontier
  Fields Galaxy Cluster Abell 370
  We present a detailed weak-lensing and X-ray study of the Frontier Fields
galaxy cluster Abell 370, one of the most massive known lenses on the sky,
using wide-field BRz Subaru/Sprime-Cam and Chandra X-ray observations. By
combining 2D shear and azimuthally averaged magnification constraints derived
from Subaru data, we perform a lensing mass reconstruction in a free-form
manner, which allows us to determine both radial structure and 2D morphology of
the cluster mass distribution. In a triaxial framework assuming an NFW density
profile, we constrain the intrinsic structure and geometry of the cluster halo
by forward modeling the reconstructed mass map. We obtain a halo mass
$M_{200}=(1.54 \pm 0.29)\times 10^{15}h^{-1}M_\odot$, a halo concentration
$c_{200}=5.27 \pm 1.28$, and a minor-major axis ratio $q_a=0.62 \pm 0.23$ with
uninformative priors. Using a prior on the line-of-sight alignment of the halo
major axis derived from binary merger simulations constrained by multi-probe
observations, we find that the data favor a more prolate geometry with lower
mass and lower concentration. From triaxial lens modeling with the
line-of-sight prior, we find a spherically enclosed gas mass fraction of
$f_\mathrm{gas}=(8.4 \pm 1.0)\%$ at $0.7h^{-1}$ Mpc. When compared to the
hydrostatic mass estimate from Chandra observations, our triaxial weak-lensing
analysis yields spherically enclosed mass ratios of $1-b=0.56 \pm 0.09$ and
$0.51 \pm 0.09$ at $0.7h^{-1}$ Mpc with and without using the line-of-sight
prior, respectively. Since the cluster is in a highly disturbed dynamical
state, this represents the likely maximum level of hydrostatic bias in galaxy
clusters. We also obtain a model-independent constraint of
$M_\mathrm{2D}(<2.3h^{-1}\mathrm{Mpc})=(3.11 \pm 0.47)\times
10^{15}h^{-1}M_\odot$ for the projected mass of the whole system, including any
currently unbound material around the cluster.
