Suzaku and XMM-Newton Observations of the North Polar Spur: Charge Exchange or ISM Absorption?

TL;DR

Revisiting Suzaku and XMM-Newton data reveals that the North Polar Spur's spectra are better explained by ionized absorption rather than neutral absorption, indicating it is a Galactic halo structure with specific plasma properties.

Contribution

This study introduces an ionized absorption model to explain spectral features of the North Polar Spur, challenging previous neutral absorption assumptions.

Findings

The plasma is consistent with a single-phase CIE component at 0.25 keV.

Ionized absorption characterized by a temperature of 0.17-0.20 keV and a column density of 3-5 x 10^{19} cm^{-2}.

Abundance pattern aligns with Galactic halo stars.

Abstract

By revisiting the Suzaku and XMM-Newton data of the North Polar Spur, we discovered that the spectra are inconsistent with the traditional model consisting of pure thermal emission and neutral absorption. The most prominent discrepancies are the enhanced O VII and Ne IX forbidden-to-resonance ratios, and a high O VIII Ly$\beta$ line relative to other Lyman series. A collisionally ionized absorption model can naturally explain both features, while a charge exchange component can only account for the former. By including the additional ionized absorption, the plasma in the North Polar Spur can be described by a single-phase CIE component with temperature of 0.25 keV, and nitrogen, oxygen, neon, magnesium, and iron abundances of $0.4-0.8$ solar. The abundance pattern of the North Polar Spur is well in line with those of the Galactic halo stars. The high nitrogen-to-oxygen ratio reported in previous studies can be migrated to the large transmission of the O VIII Ly$\alpha$ line. The ionized absorber is characterized by a balance temperature of $0.17-0.20$ keV and a column density of $3-5 \times 10^{19}$ cm$^{-2}$. Based on the derived abundances and absorption, we speculate that the North Polar Spur is a structure in the Galactic halo, so that the emission is mostly absorbed by Galactic ISM in the line of sight.