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 , O VII Kˇ, O VIII K , and Ne IX K 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) 10 K for the front-side plasma and T = (3.9 0:3) 10 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.