Discovery of non-equilibrium ionization plasma associated with the North
  Polar Spur and Loop I
  We investigated the detailed plasma condition of the North Polar Spur
(NPS)/Loop I using archival $Suzaku$ data. In previous research collisional
ionization equilibrium (CIE) have been assumed for X-ray plasma state, but we
also assume non-equilibrium ionization (NEI) to check the plasma condition in
more detail. We found that most of the plasma in the NPS/Loop I favors the
state of NEI, and has the density-weighted ionization timescale of $n_e
t\sim10^{11-12}$ s cm$^{-3}$ and the electron number density $n_e\sim$ a few
$\times$ 10$^{-3}$ cm$^{-3}$. The plasma shock age, $t$, or the time elapsed
after the shock front passed through the plasma, is estimated to be on the
order of a few $\rm{Myr}$ for the NPS/Loop I, which puts a strict lower limit
to the age of the whole NPS/Loop I structure. We found that NEI results in
significantly higher temperature and lower emission measure than those
currently derived under CIE assumption. The electron temperature under NEI is
estimated to be as high as 0.5 keV toward the brightest X-ray NPS ridge at
$\Delta\theta=-20^\circ$, which decreases to 0.3 keV at $-10^\circ$, and again
increases to $\sim 0.6$ keV towards the outer edge of Loop I at
$\Delta\theta\sim0^\circ$, about twice the currently estimated temperatures.
Here, $\Delta \theta$ is the angular distance from the outer edge of Loop I. We
discuss the implication of introducing NEI for the research in plasma states in
astrophysical phenomena.
