An X-ray Spectroscopic Study of the Hot Interstellar Medium Toward the Galactic Bulge

TL;DR

This study combines emission and absorption X-ray spectroscopy to characterize the hot interstellar medium toward the Galactic bulge, revealing temperature differences consistent with a hot ISM disk and bulge.

Contribution

It provides a detailed spectroscopic analysis of the hot gas toward the Galactic bulge using combined emission and absorption data, offering new insights into the temperature structure.

Findings

Identified two plasma components with different temperatures.

Confirmed the presence of hot gas consistent with a thick ISM disk.

Supported the existence of an X-ray bulge around the Galactic center.

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 OVII Kalpha, OVII Kbeta, OVIII Kalpha and NeIX Kalpha by Chandra LTGS as shown by previous works, Suzaku detected clearly the emission lines of OVII, OVIII, NeIX and NeX from the vicinity. We used simplified plasma models with constant temperature and density. Evaluation of the background and foreground emission was performed carefully, including stellar X-ray contribution based on the recent X-ray observational results and stellar distribution simulator. If we assume that one plasma component exists in front of 4U1820-303 and the other one at the back, the obtained temperatures are T= 1.7 +/- 0.2 MK for the front-side plasma and T=3.9(+0.4-0.3) MK for the backside. This scheme is consistent with a hot and thick ISM disk as suggested by the extragalactic source observations and an X-ray bulge around the Galactic center.