Broad Fe K' lines in active galactic nuclei - reflection or absorption? Encouraging constraints from X-ray polarimetry
Résumé
A significant number of X-ray bright, type-1 active galactic nuclei show broad Fe K' emission lines. The origin of the broad red wing is attributed to relativistic blurring occurring very close to the innermost stable orbit and thus putting tight constraints on the spin of the black hole. Nonetheless, the broad Fe K' line of the well-studied Seyfert-1 galaxy MCG-6-30-15 has also been modeled by non-relativistic, complex absorption in a wind. Neither spectral nor timing analyzes of present day X-ray data allowed us to distinguish between the two models. In this work, we explore if X-ray polarimetry can resolve the controversy. Using our radiative transfer methods that include the effects of X-ray polarization, we explore a series of absorption and reflection models. It turns out that the polarization spectrum in the 2-10 keV band puts robust constraints on the origin of broad Fe K' lines. We conclude that a long-look observation of MCG-6-30-15 with a small polarimetry satellite could reliably distinguish between the two scenarios.