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The Si K Edge Gas to Dust Ratio Towards the Galactic Bulge

Presentation #107.06 in the session “ISM/Galaxies/Clusters (Poster)”.

Published onApr 01, 2022
The Si K Edge Gas to Dust Ratio Towards the Galactic Bulge

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 datasets that provide pile-up free spectra at Si K. We discovered an instrumental feature affecting the Si K edge structure which we included in our analysis. We find that while gas optical depths grow fairly consistent with broadband hydrogen equivalent densities, the dust optical depths do not. While some depths can be explained with an empirical limb darkening model, many other dust depths in the sample clearly cannot. This shows that the gas to dust optical depth ratio towards sources in the Galactic Budge is not isotropic. We attempt to compare the dust depth to measured molecular hydrogen columns. We also fit ionized Si K contributions towards the Galactic Bulge and while we do not find any correlation lowly ionized Si K absorption with source luminosity, we find Si XIII absorption with velocity widths of 800–1100 km/s which we attribute to the circumbinary medium.

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