Determine the velocity scales probed by resonant scattering

Determine how the additional line-of-sight ion-density weighting in resonant-scattering measurements affects the spatial scales of intracluster-medium gas motions probed by resonant scattering relative to those probed by Doppler line broadening.

Background

The paper compares velocity constraints derived from resonant-scattering line ratios with velocity dispersions measured through Doppler broadening. Doppler broadening is weighted by the X-ray emissivity, proportional to the square of the electron density, whereas resonant scattering is additionally weighted by the ion density along the line of sight, proportional to the electron density.

Because these different weightings may cause the two techniques to probe different spatial scales, the relationship between their inferred velocity amplitudes is not fully established. Resolving this issue is important for interpreting agreement between resonant-scattering and line-broadening measurements as evidence for predominantly small-scale random motions.

References

However, the RS effect is additionally weighted by the ion density along the LoS, which is $\propto n_e$. How this additional weighting affects the scales probed by RS is not immediately clear and is beyond the scope of this work.

— XRISM observations of the Perseus cluster along two arms: Chaotic ICM motions probed by resonant scattering  (2609.10845 - Heinrich et al., 9 Sep 2026) in Section 4.1, “Resonant scattering simulations: isotropic gas motions”

However, we cannot rule out the possibility of unresolved substructures in the ICM that could affect the RS signal.

— XRISM observations of the Perseus cluster along two arms: Chaotic ICM motions probed by resonant scattering  (2609.10845 - Heinrich et al., 9 Sep 2026) in Section 4.3, “Systematic uncertainties,” subsection “Spherical symmetry”