XRISM catches a hypergiant's wind plume falling onto the pulsar GX 301-2

For years the link between the flares and the plume rested on timing. GX 301-2 flares near the two ends of its orbit, most strongly at closest approach ^[11]. That pattern fits a neutron star crossing a dense stream of gas. Timing, though, only records where the star was when it brightened. Researchers had long suspected the plume, but according to Live Science they had no way of proving it without more evidence ^[12].

XRISM gave them a measurement of the gas itself. The team observed the system for 16 hours while a flare was under way ^[13]. The spectra showed "rapidly changing emission and absorption lines" that tracked the plume as it ebbed and flowed around GX 301-2, NASA representatives wrote ^[13]. "We could see how the dense stream of plasma acts very close to the neutron star," said co-author Nazma Islam, an astronomer at the Manipal Centre for Natural Sciences in India ^[14]. She added: "It was clear that these observations were groundbreaking, but at the same time this meant the analysis had to be especially detailed." ^[15]

Roi Rahin, the first author, is at the University of Maryland, Baltimore County and NASA's Goddard Space Flight Center. "We've never before seen clear indications of wind plasma falling onto a compact object," he said ^[7]. Live Science describes the result as the first detection of this kind of stellar material interacting with a neutron star or any similar remnant ^[6]. "We can now test our understanding of these processes in much greater detail," Rahin said ^[8].

The denominator here is one flare. Sixteen hours is about 1.6% of the 41.5-day orbit ^[16]. Live Science's summary says the star flared "every time" it passed through the plume ^[5]. The observation it describes covers a single event, though, and the article does not say whether XRISM caught the strong flare at closest approach or the one at the far end of the orbit. As reported, the "every time" leans on the older timing pattern ^[11]. Whether the far-point flare comes from the same plume crossing is still open.

The claim that the two objects feed each other is less settled. Wray 977 is roughly 60 times larger than the sun ^[1]. It sends its wind out as a single concentrated plume moving at around 540,000 km/h ^[9]. Scientists think the neutron star's gravity is what gathers the wind into that plume ^[10]. If so, GX 301-2, about a solar mass packed into roughly 20 km ^[2], shapes the stream that then lights it up. Live Science presents that sculpting as a likely explanation. The XRISM spectra, as described, show what the plasma does near the neutron star; they do not show how the plume formed ^[13][10].