Probing the origin of the iron K_alpha line around stellar and
  supermassive black holes using X-ray polarimetry
  Asymmetric, broad iron lines are a common feature in the X-ray spectra of
both X-ray binaries (XRBs) and type-1 Active Galactic Nuclei (AGN). It was
suggested that the distortion of the Fe K_alpha emission results from Doppler
and relativistic effects affecting the radiative transfer close to the strong
gravitational well of the central compact object: a stellar mass black hole
(BH) or neutron star (NS) in the case of XRBs, or a super massive black hole
(SMBH) in the case of AGN. However, alternative approaches based on
reprocessing and transmission of radiation through surrounding media also
attempt to explain the line broadening. So far, spectroscopic and timing
analyzes have not yet convinced the whole community to discriminate between the
two scenarios. Here we study to which extent X-ray polarimetric measurements of
black hole X-ray binaries (BHXRBs) and type-1 AGN could help to identify the
possible origin of the line distortion. To do so, we report on recent
simulations obtained for the two BH flavors and show that the proposed
scenarios are found to behave differently in polarization degree and
polarization angle. A relativistic origin for the distortion is found to be
more probable in the context of BHXRBs, supporting the idea that the same
mechanism should lead the way also for AGN. We show that the discriminating
polarization signal could have been detectable by several X-ray polarimetry
missions proposed in the past.
