The Presence of Dust and Ice Scattering in X-Ray Emissions from Comets

TL;DR

This study models X-ray emissions from comets, showing charge-exchange dominates below 1 keV while dust and ice scattering are significant above 1 keV, with grain size distributions varying between comets.

Contribution

It introduces a comprehensive model combining charge-exchange and scattering mechanisms, validated against Chandra observations, revealing the importance of dust and ice scattering above 1 keV.

Findings

Charge-exchange reproduces spectra below 1 keV.

Dust and ice scattering dominate above 1 keV.

Grain size distribution varies between comets.

Abstract

X-ray emissions from cometary atmospheres were modeled from first principles using the charge-exchange interaction with solar wind ions as well as coherent scattering of solar X-rays from dust and ice grains. Scattering cross-sections were interpolated over the 1 nm-1 cm grain radius range using approximations based on the optically thin or thick nature of grains with different sizes. The theoretical emission model was compared to Chandra observations of Comets ISON and Ikeya-Zhang due to their high signal-to-noise ratios and clearly defined spectral features. Comparing the observed intensities to the model showed that the charge-exchange mechanism accurately reproduced the emission spectra below 1 keV, while dust and ice scattering was negligible. Examining the 1-2 keV range found dust and ice scattering emissions to agree well with observations, while charge-exchange contributions were insignificant. Spectral features between the scattering model and observations also trended similarly over the 1-2 keV range. The dust and ice density within the cometary atmosphere $n$ was varied with respect to grain size $a$ as the function $n(a) \propto a^{-\alpha}$, with Ikeya-Zhang requiring $\alpha = 2.5$ and ISON requiring $\alpha = 2.2$ to best fit the observed spectral intensities. These grain size dependencies agreed with independent observations and simulations of such systems. The overall findings demonstrate evidence of significant scattering emissions present above 1 keV in the analyzed cometary emission spectra and that the dust/ice density dependence on grain radius $a$ may vary significantly between comets.