Polarization of X-ray lines from galaxy clusters and elliptical galaxies - a way to measure tangential component of gas velocity

TL;DR

This paper investigates how gas motions in galaxy clusters affect the polarization of X-ray emission lines, proposing a method to measure the tangential component of gas velocity using polarization signals.

Contribution

It introduces a novel approach to infer gas velocities in galaxy clusters through polarization measurements of X-ray lines, considering various gas motion patterns and simulation data.

Findings

Gas motions have a minor effect on polarization degree and direction.

Polarization signals are sensitive to perpendicular gas velocities.

Next-generation X-ray polarimeters can detect these polarization signals.

Abstract

We study the impact of gas motions on the polarization of bright X-ray emission lines from the hot intercluster medium (ICM). The polarization naturally arises from resonant scattering of emission lines owing to a quadrupole component in the radiation field produced by a centrally peaked gas density distribution. If differential gas motions are present then a photon emitted in one region of the cluster will be scattered in another region only if their relative velocities are small enough and the Doppler shift of the photon energy does not exceed the line width. This affects both the degree and the direction of polarization. The changes in the polarization signal are in particular sensitive to the gas motions perpendicular to the line of sight. We calculate the expected degree of polarization for several patterns of gas motions, including a slow inflow expected in a simple cooling flow model and a fast outflow in an expanding spherical shock wave. In both cases, the effect of non-zero gas velocities is found to be minor. We also calculate the polarization signal for a set of clusters, taken from large-scale structure simulations and evaluate the impact of the gas bulk motions on the polarization signal. We argue that the expected degree of polarization is within reach of the next generation of space X-ray polarimeters.