EXAFS and Interstellar Dust

Dutch astronomers C.P. de Vries and E. Costantini, both of SRON/Utrecht University Netherlands, used the Reflection Grating Spectrometer onboard the XMM-Newton satellite to obtain high-quality X-ray spectra of Scorpius X-1, one of the brightest X-ray sources in the sky, located about 2800 parsecs from Earth. For the first time, they have found clear evidence of an extended X-ray absorption fine structure (EXAFS) signature coming from the dust seen toward a celestial source.

EXAFS is a powerful tool for studying the grains in the interstellar medium. It is based upon the phenomenon that X-ray photons can eject electrons from atoms inside solid particles, which in turn can be backscattered onto the emitting atom by atoms in their immediate neighborhood. This causes characteristic sinusoidal absorption features in the X-ray spectrum of a distant source that depend on the structure of the absorbing solid material.

EXAFS has a major advantage over infrared (IR) spectroscopy, which can also be used to study crystalline dust, in that in can probe the solid matter along the line of sight at the level of the atomic structure. IR spectroscopy provides information at the mineralogical level. As a result, EXAFS gives a more detailed picture of the chemical composition and atomic structure of amorphous grains than is possible with IR spectroscopy.