Discovery of non-equilibrium ionization plasma associated with the North Polar Spur and Loop I

Abstract

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 $n_{e}t\sim 10^{11-12}$ 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 $\mathrm{Myr}$ 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 $\Delta \theta \sim 0^{\circ }$, 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.

11 pages, 5 figs, accepted in MNRAS

References in corpus (8)

• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Detection of large-scale X-ray bubbles in the Milky Way halo
• Polarised radio filaments outside the Galactic plane
• The Structure of the Local Hot Bubble
• The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo
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