Astronomers have directly observed a pulsar capturing and feeding on the stellar wind of a massive companion star, using X-ray data from Japan's Xrism observatory. The findings, published in Science Advances, provide the first clear spectroscopic evidence of plasma from a hypergiant star falling onto a compact stellar object, confirming a process that had previously been inferred only from indirect clues.
The system, known as BP Crucis, lies roughly 13,000 light-years away in the constellation Crux, the Southern Cross. It consists of two extreme objects locked in orbit. The primary star, Wray 977, is a blue hypergiant about 60 times larger than the Sun and 40 times more massive. It is so hot, large, and luminous that it continuously sheds a stellar wind — a stream composed mostly of superheated gas known as plasma.
Its companion, Gx 301-2, is a pulsar: a rapidly spinning neutron star that rotates roughly once every 11 minutes and sweeps a beam of X-rays toward Earth like a lighthouse. The pulsar packs more mass than the Sun into a sphere only about 20 kilometers across — a density that would be comparable to compressing nearly two solar masses into the width of a large city.
As the pulsar travels along its orbit, intense X-ray flares occur at two points: when it is closest to the hypergiant and when it is farthest away. Researchers believe the pulsar's gravitational influence on the hypergiant produces a particularly dense stream of plasma, and the flares happen when the pulsar passes through this stream and captures some of the material — much like a person wading across a river of dense material and collecting some of it along the way. The most intense emissions occur near the hypergiant, where the stream is densest.
The research team pointed Xrism at the system on February 1, 2025, observing it for about 16 hours near the end of one of these stronger flares. The Resolve instrument, developed by NASA and the Japan Aerospace Exploration Agency, captured X-ray spectra that allowed scientists to determine the speed and direction of the plasma flow near the pulsar. Such observations had never been made before.
The results show that absorption lines from iron are shifted toward lower energies than they would be in a laboratory. This shift, called redshift, indicates that the gas in the stellar wind is flowing toward the pulsar. It also allowed the team to calculate the flow speed at about 540,000 kilometers per hour.
«We had never before observed clear indications of a plasma wind falling onto a compact object,» said Roi Rahin, a researcher at the University of Maryland, Baltimore County and NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the paper. «Now we can verify our understanding of these processes in greater detail.»
The team proposes the following sequence: first, the pulsar enters the stellar wind stream and gathers gas into a disordered, turbulent disk. This gas spirals toward the pulsar, heats up, and emits X-rays that power the flares. Then, as the pulsar moves deeper into the stream, the turbulent disk breaks apart, and once it has vanished, plasma falls directly onto the pulsar. The Xrism observations took place near the end of this phase. As the pulsar is about to exit the stream, a disordered disk briefly reappears, this time rotating in the opposite direction. Finally, when the pulsar leaves the stream, this disk also disappears. In total, the pulsar takes about four days to cross the plasma stream.
«The BP Crucis system is an ideal laboratory for studying wind-fed accretion onto pulsars,» said Brian Williams, project scientist for the mission at NASA Goddard Space Flight Center, «and Xrism's sensitive, high-resolution Resolve spectrometer is the ideal instrument to advance our knowledge of the processes involved.»
The study, titled «Direct spectroscopic observation of matter falling onto a compact stellar object,» adds a key piece of evidence to models of how compact objects accrete matter from massive stars — a mechanism that shapes the evolution of binary systems and the production of X-ray binaries across the universe.