Cosmic Wind Plume Triggers Repeated X-ray Flares in Neutron Star

A massive, ongoing stream of stellar material is responsible for triggering repeated X-ray outbursts from a distant neutron star, according to new research. The findings offer unprecedented insight into how extreme celestial objects interact within binary systems.

Located approximately 13,000 light-years from Earth, the BP Crucis system consists of two stars in close orbit: a blue hypergiant named Wray 977, which is roughly 60 times the size of the sun, and a neutron star designated GX 301-2. Often referred to as a “zombie star” due to its origin from a supernova explosion, this ultradense remnant contains the mass of the sun compressed into a sphere only about 12 miles (20 kilometers) wide. As a pulsar, GX 301-2 possesses an intense magnetic field and spins rapidly, emitting regular beams of electromagnetic radiation, including X-rays. When these beams align with Earth, they create a detectable flash every 11 minutes.

However, unlike typical pulsars, GX 301-2 exhibits additional, brighter X-ray flares. Until now, the cause of these intermittent surges remained unknown. In a study published September 18 in the journal Science Advances, researchers utilized the X-ray Imaging and Spectroscopy Mission (XRISM), a joint satellite project between NASA and the Japan Aerospace Exploration Agency, to investigate the system.

The data confirmed that the neutron star flares each time it passes through a massive plume of stellar wind plasma ejected by its partner, Wray 977. This represents the first time scientists have directly observed this specific type of stellar material interacting with a neutron star or similar compact remnant.

“We’ve never before seen clear indications of wind plasma falling onto a compact object,” said Roi Rahin, the study’s lead author and a researcher at 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.”

All stars undergoing nuclear fusion emit streams of charged particles known as stellar wind. While the sun produces a relatively gentle solar wind, massive stars like Wray 977 generate far more extreme outflows. This particular hypergiant emits a concentrated jet of ionized gas traveling at approximately 335,000 mph (540,000 km/h). Scientists believe the gravitational pull of the nearby neutron star may be sculpting the wind into this single, dense plume.

GX 301-2 completes an orbit around Wray 977 every 41.5 days. It experiences X-ray flares at both the closest and farthest points of its orbit, with the most intense bursts occurring at periastron, or the point of closest approach. While researchers had long hypothesized that these flares were caused by the neutron star passing through the wind plume, they lacked the evidence to prove it.

During a 16-hour observation window that captured one of the flares, XRISM recorded detailed X-ray spectra. The observations revealed rapidly shifting emission and absorption lines, illustrating how the plasma plume ebbed and flowed around the neutron star.

“We could see how the dense stream of plasma acts very close to the neutron star,” said Nazma Islam, a study co-author and astronomer at the Manipal Centre for Natural Sciences in India. “It was clear that these observations were groundbreaking, but at the same time this meant the analysis had to be especially detailed.”

Using this data, the team simulated the accumulation and swirling of stellar material on the neutron star’s surface. When sufficient material builds up, it triggers an explosive reaction, producing the bright flashes detected on Earth.

The researchers plan to use XRISM to monitor future flare events to further unravel the dynamics between the two stars. Brian Williams, XRISM project scientist at NASA Goddard, described the BP Crucis system as an ideal laboratory for studying wind-fed pulsar accretion and noted that the mission is well-suited to advancing understanding of these complex processes.