Astronomers using the XRISM observatory have recorded gas plunging toward the X-ray pulsar GX 301-2 at speeds of at least 540,000 kilometers per hour as it crosses the stellar wind of blue hypergiant Wray 977.
Using data from the Japan-led X-ray Imaging and Spectroscopy Mission observatory, researchers have captured details of a giant star’s outflow being reeled in by a compact stellar companion. The observations focus on BP Crucis, a high-mass X-ray binary situated about 13,000 light-years away in the southern constellation Crux, as detailed in a study published in Science Advances.
BP Crucis Binary System Mechanics and Stellar Wind Capture
The primary star in the binary system, Wray 977, is a blue hypergiant about 40 times the mass of the Sun and 60 times its size. Due to its extreme temperature and luminosity, ionized gas streams away from the hypergiant continuously, forming a stellar wind. Its companion is a neutron star designated GX 301-2, which packs more than a solar mass into a dense sphere approximately 12 miles across. Rotating on an 11-minute cycle, the neutron star sweeps an X-ray beam toward Earth, earning its classification as a pulsar.
Twice during its 41.5-day orbit, the pulsar experiences multi-day X-ray flares near its closest and farthest points from the primary star. Gravitational pull from the pulsar creates an especially dense stream of plasma within the hypergiant’s wind. Strongest eruptions occur close to the primary star where the outflow is densest.
Resolve Instrument Spectra Reveal Rapidly Changing Plasma Velocity
The research team targeted the celestial system with the XRISM observatory on February 1, 2025, monitoring it for roughly 16 hours near the tail end of a strong X-ray flare. The observatory’s Resolve instrument—jointly developed by NASA and the Japan Aerospace Exploration Agency—captured high-resolution X-ray spectra that displayed rapidly shifting emission and absorption lines.
Absorption lines originating from highly ionized iron showed shifts toward lower energies than identical atomic transitions measured in a laboratory setting. This Doppler redshift indicates that the observed gas was moving away from Earth and flowing toward the neutron star.
Initial spectral readings prompted extensive cross-checking against existing scientific literature. Co-author Nazma Islam noted that the uniqueness of the data demanded rigorous analytical verification to map how plasma interacts in close proximity to a neutron star.
Disruption and Reversal of the Accretion Disk
Analysis of the iron lines demonstrated that gas raced toward the pulsar at approximately 335,000 mph (540,000 kph). According to the research team, velocity calculations based on adopted ephemerides confirm an inflow speed ranging from 150 to 180 kilometers per second, making the commonly cited figure a lower bound.
- As the pulsar enters the stellar wind, captured gas initially forms a messy, turbulent accretion disk that spirals inward.
- Deeper inside the dense stream, incoming material loses the necessary angular momentum to maintain the disk structure, causing it to break apart.
- Deprived of the disk, plasma falls in a more radial stream directly onto the neutron star.
- Near the far edge of the stream, a new accretion disk briefly re-forms, rotating in the opposite direction due to the overarching flow of the stellar wind.
While long-term data like the 17 years of Fermi timing points provide supporting torque-sign evidence consistent with opposite angular momentum, the XRISM observation captured a critical temporal sequence near the end of a flare phase rather than a complete recording of the disk reversal.
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