Weak-Lensing Mass Measurements of Five Galaxy Clusters in the South Pole
  Telescope Survey Using Magellan/Megacam
  We use weak gravitational lensing to measure the masses of five galaxy
clusters selected from the South Pole Telescope (SPT) survey, with the primary
goal of comparing these with the SPT Sunyaev--Zel'dovich (SZ) and X-ray based
mass estimates. The clusters span redshifts 0.28 < z < 0.43 and have masses
M_500 > 2 x 10^14 h^-1 M_sun, and three of the five clusters were discovered by
the SPT survey. We observed the clusters in the g'r'i' passbands with the
Megacam imager on the Magellan Clay 6.5m telescope. We measure a mean ratio of
weak lensing (WL) aperture masses to inferred aperture masses from the SZ data,
both within an aperture of R_500,SZ derived from the SZ mass, of 1.04 +/- 0.18.
We measure a mean ratio of spherical WL masses evaluated at R_500,SZ to
spherical SZ masses of 1.07 +/- 0.18, and a mean ratio of spherical WL masses
evaluated at R_500,WL to spherical SZ masses of 1.10 +/- 0.24. We explore
potential sources of systematic error in the mass comparisons and conclude that
all are subdominant to the statistical uncertainty, with dominant terms being
cluster concentration uncertainty and N-body simulation calibration bias.
Expanding the sample of SPT clusters with WL observations has the potential to
significantly improve the SPT cluster mass calibration and the resulting
cosmological constraints from the SPT cluster survey. These are the first WL
detections using Megacam on the Magellan Clay telescope.
