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