Ann Arbor Times

An international team of scientists has achieved a significant milestone in the study of interstellar sulfur using data from the Japan-led X-ray Imaging and Spectroscopy Mission (XRISM) spacecraft. This research marks the first direct measurement of sulfur's gas and solid phases, thanks to XRISM's unique X-ray spectroscopy capabilities.

“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 Lía Corrales, assistant professor of astronomy at the University of Michigan.

The team used X-rays from two binary star systems to detect sulfur in the interstellar medium, which consists of gas and dust between stars. “Sulfur can easily change from a gas to a solid and back again,” Corrales explained. “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.”

A paper detailing these findings was published on June 27 in the Publications of the Astronomical Society of Japan. Researchers previously detected gaseous sulfur using ultraviolet light but found that this form quickly disappears in denser parts like molecular clouds, suggesting that sulfur condenses into solids by combining with ice or other elements.

Corrales and her team selected an area with appropriate density for their study, using a bright X-ray source behind this section called GX 340+0. The Resolve instrument on XRISM allowed them to measure GX 340+0’s X-rays, confirming that sulfur exists as both gas and solid, possibly mixed with iron.

“Chemistry in environments like the interstellar medium is very different from anything we can do on Earth," said co-author Elisa Costantini from Space Research Organization Netherlands and University of Amsterdam. "Our lab has created models for different elements to compare with astronomical data for years."

In their publication, Corrales' team suggests compounds such as pyrrhotite, troilite, and pyrite could match XRISM’s observations. They also utilized measurements from another binary system called 4U 1630-472 to support their conclusions.

“NASA’s Chandra X-ray Observatory has previously studied sulfur, but XRISM’s measurements are the most detailed yet,” stated Brian Williams from 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.”

The mission is led by JAXA (Japan Aerospace Exploration Agency) with collaboration from NASA and contributions from ESA (European Space Agency). NASA and JAXA jointly developed Resolve, XRISM's microcalorimeter spectrometer.