Data from the Japan-led X-ray Imaging and Spectroscopy Mission (XRISM) has provided the first direct observation of stellar wind plasma being captured by a compact companion star. The findings, published in Science Advances, detail the interaction within the BP Crucis system, a high-mass X-ray binary located 13,000 light-years away in the constellation Crux.
The system consists of Wray 977, a blue hypergiant approximately 40 times the mass of the Sun, and its companion, a neutron star known as GX 301-2. The neutron star, which functions as a pulsar, orbits the hypergiant every 41.5 days. During this orbit, the pulsar experiences intense X-ray flares as it passes through a dense stream of plasma generated by the hypergiant’s stellar wind.
Using the Resolve instrument—a collaborative development between NASA and the Japan Aerospace Exploration Agency (JAXA)—researchers observed the system for 16 hours on February 1, 2025. The high-resolution X-ray spectra revealed iron absorption lines shifted to lower energies, a phenomenon known as redshift. This data allowed the team to calculate that gas is accelerating toward the pulsar at approximately 335,000 mph.
The research team suggests that as the pulsar enters the plasma stream, it forms a turbulent disk of gas that spirals inward, generating X-rays. As the pulsar moves deeper into the flow, the disk dissipates, causing the plasma to fall directly onto the neutron star. This process repeats in reverse as the pulsar exits the stream.
"We’ve never before seen clear indications of wind plasma falling onto a compact object," said Roi Rahin, a researcher at the University of Maryland, Baltimore County (UMBC) and NASA’s Goddard Space Flight Center. The study provides a new opportunity to test existing models of accretion processes in extreme cosmic environments.
Source: NASA
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