Using the X-ray spacecraft XRISM (X-ray Imaging and Spectroscopy Mission), astronomers have observed stellar winds from a hypergiant star feeding an extreme stellar remnant known as a neutron star, left over after a massive star dies.
Neutron stars are considered so extreme because the process that creates them crams between one and two times the mass of the sun down into a sphere with a width of around 12 miles (20 kilometers). This means that the matter that comprises neutron stars is so dense that if a teaspoon of it were brought to Earth, it would weigh as much as 10 million tons, or about the same as 85,000 adult blue whales. In addition to this some young neutron stars, known as pulsars, spin as rapidly as 700 times per second while also blasting twin beams of radiation that sweep across the cosmos like beams of light from a cosmic lighthouse.
Using XRISM and focusing on the system BP Crucis, located about 13,000 light-years away, astronomers investigated what happens when stellar winds from a blue hypergiant star dump ionized gas or plasma on the surface of a pulsar called GX 301-2. The star in question is Wray 977, which has around 40 times the sun's mass and 60 times its size. This means Wray 977 is so hot that it constantly emits a stream of plasma.
GX 301-2 rotates once every 11 minutes and takes around 42 Earth days to orbit its blue hypergiant stellar companion. The team theorizes that the immense gravity of the neutron star creates a dense stream of plasma that it passes through during its orbit, causing flares of X-rays. These flares are more extreme when the neutron star is closest to its stellar donor and thus passing through a denser stream of plasma.
XRISM honed in on BP Crucis using its Resolve instrument in February 2025 for around 16 hours, catching the tail end of one of these extreme flaring events.
From these observations, the team discovered the speed and direction of the plasma stream bombarding GX 301-2. They found the plasma stream is travelling at 335,000 miles per hour (540,000 kilometers per hour), around 200 times faster than the top speed of a Lockheed Martin F-16 jet fighter here on Earth.
"It was clear that these observations were groundbreaking, but at the same time this meant the analysis had to be especially detailed," Nazma Islam, a researcher at the Manipal Center for Natural Sciences, India, said in a statement. "We could see how the dense stream of plasma acts very close to the neutron star."
What's going on around this extreme dead star?
The team theorizes that as the neutron star enters the stellar stream, it sweeps plasma toward itself, creating a thick disk of matter around the stellar remnant. The gas is fed from the disk to the neutron star, causing it to heat up and emit X-rays.
The disk fragments as the neutron star passes further into the densest region of the river of plasma. That is because the matter at this point no longer has enough angular momentum to maintain the disk.
With the disk gone, though it briefly reappears, albeit spinning in the opposite direction, the plasma can directly hit the neutron star's surface, the process that creates the most intense X-ray flaring. The neutron star moves completely through the stream over a period of four Earth days.
"The BP Crucis system is an ideal laboratory for studying wind-fed pulsar accretion, and XRISM's sensitive, high-resolution Resolve spectrometer is an ideal instrument for advancing our understanding of the processes involved," said Brian Williams, the mission's project scientist at NASA's Goddard Space Flight Center.
The team's research was published on Friday (Sept. 18) in the journal Science Advances.
