XRISM Sees Pulsar Gathering Companion's Wind in BP Crucis

TL;DR: Japan-led XRISM directly caught wind plasma from blue hypergiant Wray 977 falling onto pulsar GX 301-2 at about 335,000 mph during an X-ray flare in BP Crucis.

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 finding was reported Sept. 18, 2026, by NASA Science in “NASA-JAXA XRISM Mission Sees Pulsar Gathering Companion’s ‘Wind’”, with a paper describing the results published Friday in the journal Science Advances.

According to NASA Science, researcher Roi Rahin, affiliated with UMBC (University of Maryland, Baltimore County) and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said the team had seen clear indications of wind plasma falling onto a compact object for the first time. Rahin said the team can now test understanding of these processes in much greater detail. NASA describes the research as part of its exploration of the extreme universe to better understand how the cosmos works.

The peer-reviewed record frames the same event as direct spectroscopic evidence of inflow. The abstract of “Direct spectroscopic observation of matter falling onto a compact stellar object” in Science Advances reports redshifted hydrogen- and helium-like iron lines, together with transitions from lower ionization states, observed with XRISM/Resolve during an X-ray flare. That abstract states that the features evolved from absorption lines at the beginning of the observation to emission lines at the end, with intervening phases showing a mixture of both, constituting direct evidence of inflowing material accreting onto the neutron star.

A mismatched binary 13,000 light-years away

The target system is BP Crucis, a high-mass X-ray binary located about 13,000 light-years away in the southern constellation Crux, according to NASA Science. Its primary star, known as Wray 977, is a blue hypergiant about 40 times the Sun’s mass and 60 times its size. Because the star is so big, hot, and luminous, ionized gas constantly streams away from it, a phenomenon astronomers call a stellar wind.

The companion is a neutron star called GX 301-2. According to NASA Science, it is the crushed core of a star that long ago exploded as a supernova, packing more than the Sun’s mass into a ball roughly 12 miles (20 kilometers) across. It rotates every 11 minutes and sweeps an X-ray beam toward Earth, which classifies it as a pulsar.

The pairing creates a repeating interaction. 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, NASA Science reports. 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.

NASA’s Scientific Visualization Studio, in its Sept. 18, 2026 visualization release for the result, describes the same configuration: a blue hypergiant 40 times the Sun’s mass, a pulsar passage through its dense plasma stream, and a flare in X-ray brightness as the pulsar pulls in gas. The studio’s four-day passage outline matches the NASA Science account of flare duration and geometry.

A 16-hour stare on Feb. 1, 2025

The researchers targeted the system with XRISM on Feb. 1, 2025, observing it for about 16 hours near the end of one of the stronger flares, according to NASA Science. 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.

In particular, absorption lines from highly ionized iron revealed the speed and direction of plasma relatively close to the pulsar. When Rahin first saw these spectra, NASA Science reports, he realized he had not seen anything like them before. He searched the scientific literature for similar observations and came up empty-handed.

Nazma Islam, a co-author formerly at UMBC and NASA Goddard and now an assistant professor at Manipal Centre for Natural Sciences, India, said the observations were groundbreaking and therefore required especially detailed analysis. Islam said the team could see how the dense stream of plasma acts very close to the neutron star, according to NASA Science.

The Science Advances author list identifies Roi Rahin of Goddard Space Flight Center as corresponding author, with co-authors including Ralf Ballhausen, Nazma Islam, Maurice A. Leutenegger, Ehud Behar, Daisy Zamora, Joel B. Coley, Natalie Hell, T. R. Kallman, Maximilian Lorenz, Pragati Pradhan, J. Wilms and A. Zainab. The paper’s funding acknowledgment cites NASA, the U.S. Department of Energy and the Israel Science Foundation.

Redshifted iron shows gas racing inward

Rahin and his team show that the iron absorption lines observed are displaced to lower energies than they would be if measured in a laboratory, according to NASA Science. This displacement, called a redshift, indicates motion away from the observer, which means the gas is flowing toward the pulsar. The extent of the redshift indicates the plasma’s velocity.

The team’s analysis indicates gas racing toward the pulsar at speeds of around 335,000 mph (540,000 kph). NASA’s Scientific Visualization Studio reports the same velocity and explains that prominent iron absorption lines shifted to lower energies indicate both direction and velocity. Its caption notes that data and error bars are shown in gray, with a model spectrum in light blue, and that Roman numerals indicate the ionization state of iron atoms.

The Science Advances abstract adds an important time sequence: the spectral features evolve from absorption at the beginning to emission at the end, with mixed absorption and emission in between. The authors interpret that behavior as direct evidence of inflowing material and describe a dynamic accretion geometry evolving from predominantly radial inflow to a transverse, potentially disk-like configuration during the flare.

Messy disks that form, break and reverse

NASA Science outlines what the researchers think happens during the roughly four-day transit of the stream. As the pulsar enters the stream, it sweeps up gas into a thick, messy, turbulent disk. That gas spirals down to the pulsar, heats up, and emits X-rays that power the flares.

As the pulsar pushes farther into the stream, the turbulent disk breaks down. Astronomers suspect that, as the pulsar moves more directly into the flow, the stream no longer has the angular momentum required to maintain the disk. Once the disk dissipates, plasma flows directly onto the neutron star. NASA Science states that the XRISM observations occurred near the end of this direct-inflow phase.

Then, as the pulsar nears the end of the stream, a messy disk briefly returns, this time spinning in the opposite direction from the earlier disk because of the stream’s flow. That second disk also disappears as the pulsar exits. In all, the pulsar takes about four days to transit the stream, according to the NASA Science and Scientific Visualization Studio descriptions.

The visualization release breaks the same passage into animated segments: gas flowing past the pulsar first forms a clockwise-spinning accretion disk, the disk breaks up and gas flows directly onto the pulsar, a new counterclockwise-spinning disk forms near the end of the passage, and the disk then breaks up as the pulsar exits.

Why direct inflow matters

The central advance is observational directness. Earlier work had associated GX 301-2 flares with passage through dense circumstellar material, but XRISM/Resolve captured redshifted iron lines changing through the flare in a way the authors identify as inflow onto the neutron star. According to NASA Science, no comparable wind-plasma infall signature on a compact object had been clearly seen before.

The Science Advances abstract states that observations of inflowing matter provide a direct test of accretion hypotheses and highlight the interaction between stellar wind and transient accretion disks in wind-accreting systems. For BP Crucis, that means the same 16-hour data set constrains both the inward speed close to the pulsar and the changing geometry from radial inflow to a transverse, potentially disk-like configuration.

For XRISM, the result also demonstrates the role of high-resolution X-ray spectroscopy. Resolve’s ability to separate rapidly changing emission and absorption features, including highly ionized iron lines, allowed the team to track motion toward the pulsar rather than inferring capture only from brighter X-ray output. NASA and JAXA jointly developed Resolve for that spectroscopic task on the Japan-led mission.