X-ray polarimetry as a new tool to discriminate reflection from
  absorption scenarios -- Predictions for MCG-6-30-15
  We present modelling of X-ray polarisation spectra emerging from the two
competing scenarios that are proposed to explain the broad Fe K{\alpha} line in
the Seyfert 1 galaxy MCG-6-30-15. The polarisation signature of complex
absorption is studied for a partial covering scenario using a clumpy wind and
compared to a reflection model based on the lamp-post geometry. The shape of
the polarisation percentage and angle as a function of photon energy are found
to be distinctly different between the reflection and the absorption case.
Relativistic reflection produces significantly stronger polarisation in the
1-10 keV energy band than absorption. The spectrum of the polarisation angle
adds additional constraints: in the absorption case it shows a constant shape,
whereas the relativistic reflection scenario typically leads to a smooth
rotation of the polarisation angle with photon energy. Based on this work, we
conclude that a soft X-ray polarimeter on-board a small X-ray satellite may
already discriminate between the absorption and the reflection scenario. A
promising opportunity may arise with the X-ray Imaging Polarimetry Explorer
(XIPE) mission, which has been proposed to ESA in response to a small-size
(S-class) mission call due for launch in 2017.
