X-ray polarimetry as a new tool to discriminate reflection from absorption scenarios - predictions for MCG-6-30-15
Résumé
We present modelling of X-ray polarization spectra emerging from the two competing scenarios that are proposed to explain the broad Fe Kα line in the Seyfert 1 galaxy MCG-6-30-15. The polarization signature of complex absorption is studied for a partial covering scenario using a clumpy wind and compared to a reflection model based on the lamppost geometry. The shape of the polarization percentage and angle as a function of photon energy are found to be distinctly different between the reflection and the absorption cases. Relativistic reflection produces significantly stronger polarization in the 1-10 keV energy band than absorption. The spectrum of the polarization 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 polarization angle with photon energy. Based on this work, we conclude that a soft X-ray polarimeter onboard a small X-ray satellite may already discriminate between the absorption and the reflection scenarios. A promising opportunity may arise with the X-ray Imaging Polarimetry Explorer mission, which has been proposed to the European Space Agency in response to a small-size (S-class) mission call due for launch in 2017.