Non-Equilibrium Ionization State and Two-Temperature Structure in the Bullet Cluster 1E 0657−56

Abstract

Abstract We investigate a non-equilibrium ionization state and an electron–ion two-temperature structure of the intracluster medium in the merging galaxy cluster, 1E 0657$-$56 (Bullet cluster), using a series of $N$-body and hydrodynamic simulations. We find that the electron temperature at the shock layer associated with the X-ray sub-peak (bullet) is quite different depending on the thermal relaxation model between electrons and ions; $\sim $25 keV for the Coulomb thermal relaxation model and $\sim $45 keV for the instantaneous thermal relaxation model in the simulations which reproduce the observed X-ray morphology. Furthermore, both of Fe XXV and Fe XXVI are overabundant compared with the ionization equilibrium state around the shock layer, and thus, the intensity ratio between Fe XXV and Fe XXVI K$\alpha$ lines are significantly altered from that in the ionization equilibrium state. We also carry out the simulations with various sets of merger parameters, and discuss a possible range of the non-equilibrium effects in this system. Our results could be tested with future X-ray observations such as Astro-H with better sensitivity in high energy band.