XRISM Directly Observes Pulsar Feeding on Stellar Wind for First Time

Thirteen thousand light-years away in the constellation Crux, a blue hypergiant star named Wray 977 is slowly feeding its tiny companion. The companion is a neutron star—a pulsar called GX 301-2—that sweeps an X-ray beam across space every eleven minutes like a cosmic lighthouse. For decades, astronomers have theorized about what happens when a massive star's outflowing gas encounters such a compact object. Now, for the first time, they have watched it happen.

On February 1st, 2025, the XRISM observatory—a Japan-led X-ray mission—pointed its Resolve spectrometer at the BP Crucis system during one of the pulsar's intense flares. Over sixteen hours, the instrument recorded X-ray spectra with unprecedented detail, capturing the fingerprints of highly ionized iron and other elements in the gas streaming toward the neutron star. What the data revealed was the velocity and direction of plasma in the immediate vicinity of the pulsar itself, something no telescope had directly measured before. The research team, drawn from NASA's Goddard Space Flight Center, institutions in India and Israel, the U.S. Naval Academy, and several universities, published their findings in Science Advances.

The observations showed that the ionized gas was moving away from the observer's perspective—a redshift in the X-ray spectrum—at approximately 540,000 kilometers per hour. This velocity confirmed what theorists had long suspected: Wray 977, a blue hypergiant of about forty solar masses, was indeed losing gas into space, and that gas was being captured by its neutron star companion. The star is so massive and luminous that ionized material constantly streams away from its surface, a process called stellar wind. When this wind encounters the pulsar's intense gravity, something dramatic unfolds.

The pulsar acts like a cosmic vacuum cleaner, sweeping the incoming gas into a thick, turbulent disk. This accretion disk behaves much like the disks that spiral into supermassive black holes—the gas heats to extreme temperatures as it spirals inward and radiates intense X-rays. As the pulsar orbits deeper into the stellar wind stream, the disk eventually breaks apart because the incoming gas no longer has enough rotational momentum to sustain it. When that happens, plasma flows directly onto the neutron star's surface. Later, as the pulsar moves farther along its orbit and begins to exit the wind stream, a new disk briefly forms, spinning in the opposite direction before it too dissipates.

The XRISM observations captured the system at precisely the moment when the disk had collapsed and material was flowing directly onto the neutron star. Nazma Islam, a co-author on the paper now at the Manipal Centre for Natural Sciences in India, described the challenge of interpreting such detailed data: the team had to conduct especially rigorous analysis because they were witnessing something unprecedented—the dense stream of plasma in the immediate environment of a neutron star, revealed through the shifting absorption lines in the X-ray spectrum.

What makes this observation significant is not merely that it confirms a long-held theory, but that it demonstrates XRISM's capacity to resolve the extreme physics occurring near one of the universe's most violent objects. The BP Crucis system, with its massive blue hypergiant and its neutron star companion, serves as a natural laboratory for studying how pulsars feed on stellar material. Brian Williams, XRISM's project scientist at NASA Goddard, noted that the observatory's sensitive, high-resolution spectrometer is precisely the tool needed to advance understanding of these processes. As astronomers continue to use XRISM to study similar systems across the galaxy, they will refine their models of how compact objects interact with their stellar companions—knowledge that applies to some of the most extreme phenomena the universe contains.