Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Polarization of X-ray lines from galaxy clusters and elliptical galaxies
  - a way to measure tangential component of gas velocity   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.

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The categories of this paper are: chem-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: patt-sol
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The categories of this paper are: quant-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: eess
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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom
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The categories of this paper are: q-fin
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The categories of this paper are: adap-org
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The categories of this paper are: plasm-ph
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The categories of this paper are: supr-con
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The categories of this paper are: nlin
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The categories of this paper are: stat
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The categories of this paper are: cmp-lg
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The categories of this paper are: econ