Discovery of non-equilibrium ionization plasma around the Fermi Bubble; new evidence of past activity of the Galactic center

Fermi bubbles are giant gamma-ray structures toward the Galactic center (GC) with a symmetrical north--south extension perpendicular to the galactic plane. Such giant structures toward the GC are also observed at various wavelengths from radio to X-rays, such as WMAP haze, North Polar Spur (NPS), and most recently, eROSITA bubbles. We investigated the detailed plasma condition of the NPS/Loop I around Fermi bubble using archival $Suzaku$ data. In the previous study, collisional ionization equilibrium (CIE) was assumed for the plasma state. However, we also assumed 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 that is characterized by 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$^{-3}$.