Intrinsic alignment boosting: Direct measurement of intrinsic alignments
  in cosmic shear data
  Intrinsic alignments constitute the major astrophysical systematic for
cosmological weak lensing surveys. We present a purely geometrical method with
which one can study gravitational shear-intrinsic ellipticity correlations
directly in weak lensing data. Linear combinations of second-order cosmic shear
measures are constructed such that the intrinsic alignment signal is boosted
while suppressing the contribution by gravitational lensing. We then assess the
performance of a specific parametrisation of the weights entering these linear
combinations for three representative survey models. Moreover a relation
between this boosting technique and the intrinsic alignment removal via nulling
is derived. For future all-sky weak lensing surveys with photometric redshift
information the boosting technique yields statistical errors on model
parameters of intrinsic alignments whose order of magnitude is compatible with
current constraints determined from indirect measurements. Parameter biases due
to a residual cosmic shear signal are negligible in case of quasi-spectroscopic
redshifts and remain sub-dominant for typical values of the photometric
redshift scatter. We find good agreement between the performance of the
intrinsic alignment removal based on the boosting technique and standard
nulling methods, possibly indicating a fundamental limit in the separation of
lensing and intrinsic alignment signals.
