The Si K Edge Gas and Dust Optical Depths Towards the Galactic Bulge
Abstract
Knowledge of the dust content in interstellar matter is of utmost importance in our understanding of the composition and evolution of the interstellar medium (ISM). The metal composition of the ISM enables us to study cooling and heating processes that dominate star formation rates in our Galaxy. The Chandra High Energy Transmission Grating (HETG) Spectrometer provides a unique opportunity to measure X-ray absorption of interstellar dust as well as its compositions through X-ray edge absorption structure. We measure gas and dust optical depths at Si K towards 9 bright Low-Mass X-ray Binaries (LMXBs) in the Galactic Bulge with the highest precision so far. We use HETG data sets that provide pile-up free spectra at Si K. We included a possible instrumental feature affecting the Si K edge structure in our analysis. We find that while gas optical depths grow fairly consistent with broadband hydrogen equivalent columns, the dust optical depths do not. Calculations using a formalism which includes dust self-shielding show that the observed dust optical depths can be explained by variations in dust grain columns between the various lines of sight. Variations in grain size distributions do not seem relevant. This shows that the gas to dust optical depth ratio towards sources in the Galactic Budge is not homogeneous. We also attempt to compare the dust depth to measured molecular hydrogen columns and find that dust depths roughly correlate with molecular hydrogen. Ionized Si K contributions towards the Galactic Bulge were detected but very small and while we do not find much correlation of lowly ionized Si K absorption with source luminosity, we find Si XIII absorption with velocity widths of 800 - 1100 km s-1 which we attribute to the circum-binary medium.
- Publication:
-
44th COSPAR Scientific Assembly. Held 16-24 July
- Pub Date:
- July 2022
- Bibcode:
- 2022cosp...44.2385Y