Astronomers using the XRISM observatory have directly observed stellar winds from a blue hypergiant star being captured by a spinning neutron star known as GX 301-2, revealing plasma racing at 335,000 miles per hour as it falls toward the compact dead star during X-ray flaring events.
Using the Japan-led X-ray Imaging and Spectroscopy Mission, researchers have captured details of how an extreme stellar remnant feeds on the outflow of a massive companion star. The target of the observation is the binary system BP Crucis, situated roughly 13,000 light-years away in the southern constellation Crux. The system pairs a blue hypergiant primary star named Wray 977 with a tiny, incredibly dense neutron star.
Wray 977 carries about 40 times the mass of the sun and measures 60 times its size. It burns so hot and luminous that ionized gas constantly streams away from its surface into space. Its companion, GX 301-2, is the crushed core of an ancient supernova that packs more than the sun’s mass into a sphere just 12 miles across. Rotating once every 11 minutes, the dead star sweeps an X-ray beam toward Earth, earning classification as a pulsar.
Capturing Plasma Dynamics in BP Crucis
Twice during its 41.5-day orbit around the hypergiant, the pulsar experiences strong X-ray flares that last for several days. Scientists theorize that the neutron star’s immense gravity pulls on the primary star, shaping an especially dense stream of plasma that the pulsar traverses during its passage.
On Feb. 1, 2025, researchers focused the XRISM observatory on BP Crucis for approximately 16 hours, catching the tail end of one of these intense flaring events. The spacecraft utilized its Resolve instrument—developed jointly by NASA and the Japan Aerospace Exploration Agency—to capture highly detailed X-ray spectra revealing rapidly shifting emission and absorption lines.
Analysis of the absorption lines from highly ionized iron showed that the gas is displaced to lower energies, a shift known as a redshift that indicates motion away from the observer and toward the pulsar. The data revealed that the plasma stream bombards GX 301-2 at speeds of around 335,000 miles per hour (540,000 kilometers per hour), running roughly 200 times faster than the top speed of a Lockheed Martin F-16 jet fighter.
How Accretion Disks Break Down Near the Neutron Star
The observation uncovered a turbulent cycle of matter transfer that occurs as the neutron star moves through the stellar wind. When the pulsar first enters the stream, it sweeps up gas into a thick, messy, turbulent accretion disk that spirals down to the stellar remnant, heating up and emitting the X-rays that power the flares.
As the pulsar pushes deeper into the densest region of the plasma river, the matter no longer holds enough angular momentum to maintain the disk. At this juncture, the disk fragments and dissipates, allowing plasma to fall straight onto the neutron star’s surface. Later in the passage, a new accretion disk rebuilds itself, though it spins in the opposite direction due to the ongoing flow of the stream.
The findings were published in the journal Science Advances.
Extreme Density and Future Observations
To put the extreme nature of GX 301-2 into perspective, if a single teaspoon of matter from a neutron star were brought to Earth, it would weigh approximately 10 million tons—matching the weight of about 85,000 adult blue whales. Some young neutron stars in this class also rotate up to 700 times per second.
Roi Rahin, a researcher at UMBC and NASA’s Goddard Space Flight Center, noted that the team had never before seen clear indications of wind plasma falling onto a compact object.
