Astronomers using the NASA-JAXA XRISM (X-ray Imaging and Spectroscopy Mission) observatory have directly observed plasma from a giant star's stellar wind falling onto a companion neutron star and identified this process as the power source behind intense X-ray flares. The findings were published Friday 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. Its primary star, Wray 977, is a blue hypergiant roughly 40 times the Sun's mass and 60 times its size, continuously shedding ionized gas as a stellar wind. Its compact companion, the neutron star GX 301-2, packs more than the Sun's mass into a ball roughly 12 miles (20 kilometers) across and rotates every 11 minutes, sweeping an X-ray beam toward Earth and classifying it as 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 over several days. Astronomers think the pulsar's gravity creates an especially dense plasma stream from the companion star; flares erupt as the pulsar passes through it and captures matter. The strongest eruptions occur closer to the star, where the stream is denser.

Researchers targeted BP Crucis 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 showing rapidly changing emission and absorption lines. Absorption lines from highly ionized iron were displaced to lower energies — a redshift indicating the gas is moving toward the pulsar — with the team's analysis placing the plasma's infall speed at around 335,000 mph (540,000 kph).

"We've never before seen clear indications of wind plasma falling onto a compact object," said lead researcher Roi Rahin of the University of Maryland, Baltimore County (UMBC) and NASA's Goddard Space Flight Center. "We can now test our understanding of these processes in much greater detail."

The team's model describes the pulsar sweeping captured gas into a turbulent, spiralling disk that heats up and emits X-rays. As the pulsar moves deeper into the stream, that disk breaks down and plasma flows directly onto the neutron star. Near the stream's far edge, a disk briefly reforms — this time spinning in the opposite direction — before disappearing as the pulsar exits. In total, the pulsar takes about four days to transit the stream.

Co-author Nazma Islam, formerly at UMBC and NASA Goddard and now an assistant professor at Manipal Centre for Natural Sciences in India, said the spectra were unlike anything previously documented. "We could see how the dense stream of plasma acts very close to the neutron star," she said, noting that the observations demanded especially detailed analysis.

Brian Williams, the mission's project scientist at NASA Goddard, described BP Crucis as "an ideal laboratory for studying wind-fed pulsar accretion," and XRISM's Resolve spectrometer as well-suited to advancing understanding of the processes involved.

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