Utilizing observations from two distant X-ray binary systems, astronomers detected sulfur throughout the interstellar medium-the vast mix of dust and gas between stars. XRISM's unique ability to resolve X-rays allowed scientists to trace sulfur's phase transitions, offering new insight into its cosmic distribution.
"Sulfur is important for how cells function in our bodies here on Earth, but we still have a lot of questions about where it's found out in the universe," said Lia Corrales, lead author of the study and assistant professor of astronomy at the University of Michigan. "Sulfur can easily change from a gas to a solid and back again. The XRISM spacecraft provides the resolution and sensitivity we need to find it in both forms and learn more about where it might be hiding."
Published June 27 in the Publications of the Astronomical Society of Japan, the study highlights how sulfur tends to vanish from gaseous states in dense interstellar regions, suggesting it condenses into solid forms by bonding with ice or other elements.
Corrales and her team selected the binary system GX 340+0, located over 35,000 light-years away in the constellation Scorpius, as a backlight to probe a specific section of the interstellar medium. Using XRISM's Resolve instrument, they determined that sulfur was present not only in gas form but also as a solid, potentially in compounds mixed with iron.
"Chemistry in environments like the interstellar medium is very different from anything we can do on Earth, but we modeled sulfur combined with iron, and it seems to match what we're seeing with XRISM," said co-author Elisa Costantini of the Space Research Organization Netherlands and University of Amsterdam. She added that their lab has long compared elemental models with astronomical observations and will soon compare additional sulfur samples.
The team identified iron-sulfur minerals such as pyrrhotite, troilite, and pyrite-commonly found in meteorites-as candidates matching XRISM's spectral data. Findings were further validated by data from a second X-ray binary system, 4U 1630-472.
"NASA's Chandra X-ray Observatory has previously studied sulfur, but XRISM's measurements are the most detailed yet," said Brian Williams, XRISM project scientist at NASA's Goddard Space Flight Center. "Since GX 340+0 is on the other side of the galaxy from us, XRISM's X-ray observations are a unique probe of sulfur in a large section of the Milky Way. There's still so much to learn about the galaxy we call home."
Research Report:XRISM insights for interstellar sulfur
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