An X-Ray Spectroscopic Study of the Hot Interstellar Medium toward the Galactic Bulge

Abstract

We present a detailed spectroscopic study of the hot gas toward the galactic bulge along the 4U 1820 $-$ 303 sight line by a combination analysis of emission and absorption spectra. In addition to the absorption lines of O VII K ${\alpha}$ , O VII K ${\beta}$ , O VIII K ${\alpha}$ , and Ne IX K ${\alpha}$ by Chandra LTGS, as shown by previous studies, Suzaku clearly detected the emission lines of O VII, O VIII, Ne IX, and Ne X from the vicinity. We used simplified plasma models with constant temperature and density. An evaluation of the background and foreground emission was carefully performed, including the stellar X-ray contribution based on the recent X-ray observational results and the stellar distribution simulator. If we assume that one plasma component exists in front of 4U 1820 $-$ 303 and the other one at the back, the obtained temperatures are $T$$=$ (1.7 $\ \pm\ $ 0.2) $\times$ 10 $^{6}$ K for the front-side plasma and $T$$=$ (3.9 $^{+0.4}_{-0.3}$ ) $\times$ 10 $^{6}$ K for the back-side. This scheme is consistent with a hot and thick ISM disk, as suggested by extragalactic source observations and an X-ray bulge around the galactic center.