The broad iron line in Active Galactic Nuclei: how X-ray polarimetry can distinguish between relativistic reflection and complex absorption

International audienceThe interpretation of very broad iron lines in the X-ray spectra of Active Galactic Nuclei (AGN) remains a matter of debate: on the one hand side, it is believed that the line is due to reflection happening very close to the innermost stable circular orbit (ISCO) of the accretion disk. The extended red wing of the line thus originates from gravitational and Doppler shifting and can put important constraints on the black hole spin. On the other hand, the broad iron line has also been explained by absorption processes in an outflow that partially covers the X-ray source. In this interpretation, the broadening of the line is not directly related to any relativistic effects or black hole spin. Here we explore to which extend an X-ray polarization measurement can distinguish between both models. In general, reflection polarizes light while transmission does not. Therefore, we accurately model the X-ray polarization across the iron line band for various absorption and (relativistic) reflection scenarios. We show that the behavior of the polarization degree and position angle as a function of photon energy can indeed be of great help to decide between the two scenarios