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

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 IV 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 +/- 0.2) x 10(6) K for the front-side plasma and T = (3.9(-0.3)(+0.4)) x 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.