X-ray polarimetry as a new tool to discriminate reflection from absorption scenarios -- Predictions for MCG-6-30-15

TL;DR

This study models X-ray polarisation spectra to distinguish between reflection and absorption scenarios explaining the broad Fe Kα line in galaxy MCG-6-30-15, suggesting polarimetry can effectively discriminate between these models.

Contribution

It introduces detailed polarisation predictions for both absorption and reflection models, highlighting their distinct signatures in energy-dependent polarisation spectra.

Findings

Reflection produces stronger polarisation in 1-10 keV range.

Polarisation angle remains constant in absorption but rotates in reflection.

X-ray polarimetry can differentiate between the two scenarios.

Abstract

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