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Dutch have astronomers have, for the first time, used X-ray spectroscopy to reveal the long-sought signatures of dust in the interstellar medium, the extended X-ray absorption fine structure (EXAFS). Writing in the journal Astronomy and Astrophysics, Cor de Vries of the SRON, Netherlands Institute for Space Research and Elisa Costantini of the Astronomical Institute, Utrecht University, The Netherlands, explain how they used the reflection grating spectrometer (RGS) onboard the European Space Agency's XMM-Newton satellite to obtain clear EXAFS of the astronomical X-ray sources. Until now, EXAFS studies of astronomical sources have been rather unsuccessful because only very weak X-ray signals are received from celestial objects. de Vries and Costantini studied Scorpius X-1, an X-ray source thought to be a suspected neutron star in the constellation of Scorpius, which is located more than 9000 light years from the Earth. Sco X-1 was discovered in 1962 by a team under Riccardo Giacconi at American Science and Engineering, in Cambridge, Massachusetts, and is the brightest persistent X-ray source in the sky second only to the Sun. Occasionally flaring low mass X-ray binaries may be brighter than Sco-X1, but they usually last for only a few days. For the first time, they have exposed clear evidence of an EXAFS signature coming from the interstellar dust lying between the celestial source and the Earth. EXAFS is a powerful tool for studying structure in materials science, but it is just as applicable to investigating the chemical characteristics of grains in the interstellar medium (ISM), de Vries and colleagues reasoned. It works equally as well in observing electrons ejected from from individual atoms and scattered around within the particle by X-ray photons whether they are in a ceramic sample sitting in a spectrometer on a laboratory bench as it does in observing interstellar dust. The characteristic sinusoidal absorption features in the X-ray spectrum of the sample or cosmically distant source, which change depending on the structure of the absorbing solid material can reveal much about the sample from the laboratory bench all the way to the stars. Indeed, the team reports that the spectrum around the oxygen K-edge revealed EXAFS signatures and analysis of these suggested photoelectron scattering distances in the absorbing medium in a range that applies to many solids. "When scattering of the photo-electron on oxygen atoms is assumed, the oxygen-to-oxygen atom distances found are 2.75 Å," the team explains, "This fits with distances as found in amorphous water ice, although water ice is thought to be an unlikely constituent of the diffuse interstellar clouds that form the absorbing medium towards Sco X-1." The team adds that their data will help solve this frozen conundrum once suitable laboratory data on other plausible materials become available. Previously, astronomers have focused on infrared spectroscopy as the technique of choice for probing the interstellar medium. While this is also a powerful technique for studying crystalline dust it has a major drawback when compared to EXAFS. EXAFS has the advantage over infrared spectroscopy in that it can probe the solid matter along the line-of-sight at the level of the atomic structure, even for irregular amorphous materials. IR spectroscopy operates only at the bulk, mineralogical level. As such, using EXAFS, astronomers can now obtain a very detailed sampling of the composition and atomic structure of the dust along the line-of-sight between X-ray source and the Earth. EXAFS, de Vries and colleagues explain, now that it is available to astronomers gives a much more detailed picture of the chemical composition and atomic structure of the myriad amorphous grains that lie in the interstellar medium than was possible with IR spectroscopy. The study of cosmic dust is no trivial matter. Aside from it providing astronomers with insights into the structure of the universe, the presence of molecules in the interstellar medium hinted out by such studies have implications for our understanding of the origins of life on earth. Moreover, occasionally, a molecule that falls to earth, figuratively speaking, such as buckminsterfullerene can spawn a whole new field of chemical science.
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