Does Diffuse Circumnuclear Gas around Sgr A* Achieve Collisional Ionization Equilibrium or Remain Non-equilibrium Ionization?

The feeding and feedback processes at the vicinity of a supermassive black hole(BH)are essential for our understanding of the connection between supermassive BH and its host galaxy.In this work,we provide a detailed investigation,both observational and theoretical,on the diffuse(～2"-20",～0.08-0.8 pc)X-ray emission around Sgr A*.Over two-decade Chandra observations are gathered to obtain highest signal-to-noise to date.We find that,the line center of iron lines of the outer 8"-18"region,εc=6.65+0.02-0.03 keV,is comparable to that(e=6.60+0.05-0.03 keV)of the inner 2"-5"region.This is somewhat unexpected,since the gas temperature decreases further away from the central BH.Based on a dynamical inflow-outflow model that considers the gas feeding by stellar winds from Wolf-Rayet stars,we calculate the X-ray spectrum based on both the conventional collisional ionization equilibrium(CIE)assumption,and the newly developed non-equilibrium ionization(NEI)assumption.We find that,theoretically gases within～8"-10"remain in a CIE state,outside of this radius they will be in the NEI state.A comparison of the properties of～6.6 keV iron lines between CIE and NEI is addressed.Interestingly,the NEI interpretation of outer region is supported by the Chandra line center εc measurements of this region.