Probing the origin of the iron K_alpha line around stellar and supermassive black holes using X-ray polarimetry

TL;DR

This paper investigates whether X-ray polarimetry can distinguish between relativistic effects and alternative reprocessing scenarios as the origin of broad iron K_alpha lines in black hole systems, using simulations for different black hole types.

Contribution

It demonstrates that X-ray polarimetric measurements can differentiate between the relativistic and reprocessing origins of iron line broadening in black hole systems through distinct polarization signatures.

Findings

Relativistic origin shows higher polarization degree in simulations.

Polarization angle varies differently between scenarios.

Discriminating signals could have been detected by past X-ray polarimetry missions.

Abstract

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.