This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Using data from the Japan–led XRISM (X-ray Imaging and Spectroscopy Mission) observatory, astronomers have directly observed a giant star's outflow, called a stellar wind, being captured by its compact companion and providing the power source for strong X-ray flares. The research is part of NASA's exploration of the extreme universe to better understand how the cosmos works.
"We've never before seen clear indications of wind plasma falling onto a compact object," said Roi Rahin, a researcher at UMBC (University of Maryland, Baltimore County) and NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We can now test our understanding of these processes in much greater detail." A paper describing the findings is published in the journal Science Advances. The target system is BP Crucis, a high-mass X-ray binary located about 13,000 light-years away in the southern constellation Crux.
The primary star, known as Wray 977, is a blue hypergiant about 40 times the sun's mass and 60 times its size. It's so big, hot and luminous that ionized gas constantly streams away from it, a phenomenon astronomers call a stellar wind. The supergiant's companion is a tiny-but-mighty neutron star called GX 301-2.
The crushed core of a star that long ago exploded as a supernova, it packs more than the sun's mass into a ball roughly 12 miles (20 kilometers) across. Rotating every 11 minutes, it sweeps an X-ray beam toward Earth, making it a pulsar. Twice during the pulsar's 41.5-day orbit, near its closest and farthest points from the primary star, strong X-ray flares occur for several days.
Astronomers think the pulsar's gravitational influence on the star creates an especially dense stream of plasma. Flares occur when the pulsar traverses this stream and captures some of its matter. The strongest eruptions happen closer to the star, where the stream is denser.
The researchers targeted the system with XRISM on Feb. 1, 2025, observing it for about 16 hours near the end of one of these stronger flares. The observatory's Resolve instrument, jointly developed by NASA and JAXA (Japan Aerospace Exploration Agency), captured highly detailed X-ray spectra, revealing rapidly changing emission and absorption lines. In particular, absorption lines from highly ionized iron revealed the speed and direction of plasma relatively close to the pulsar.
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