Journal Article

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

I. V. Zhuravleva, E. M. Churazov, S. Yu. Sazonov, R. A. Sunyaev, W. Forman and K. Dolag

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 403, issue 1, pages 129-150
Published in print March 2010 | ISSN: 0035-8711
Published online March 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.16148.x
Polarization of X-ray lines from galaxy clusters and elliptical galaxies – a way to measure the tangential component of gas velocity

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We study the impact of gas motions on the polarization of bright X-ray emission lines from the hot intercluster medium. 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 linewidth. 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.

Keywords: polarization; radiative transfer; scattering; methods: numerical; X-rays: galaxies: clusters

Journal Article.  15368 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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