A Detection of Weak Lensing Magnification using Galaxy Sizes and
  Magnitudes
  Weak lensing is commonly measured using shear through galaxy ellipticities,
or using the effect of magnification bias on galaxy number densities. Here, we
report on the first detection of weak lensing magnification with a new,
independent technique using the distribution of galaxy sizes and magnitudes.
These data come for free in galaxy surveys designed for measuring shear. We
present the magnification estimator and apply it to an X-ray selected sample of
galaxy groups in the COSMOS HST survey. The measurement of the projected
surface density \Sigma(r) is consistent with the shear measurements within the
uncertainties, and has roughly 40% of the signal-to-noise of the latter. We
discuss systematic issues and challenges to realizing the potential of this new
probe of weak lensing.
