A dead star has been observed swallowing part of the gas ejected by its gigantic companion.
The scene takes place in BP Crucis, a system located about 13,000 light-years from Earth. It brings together two very different stars. Wray 977 is an enormous blue star, about 40 times more massive than the Sun. Orbiting it is GX 301-2, the extremely dense core of an ancient star that exploded.
Artist’s impression of the BP Crucis system. The GX 301-2 pulsar approaches a dense stream of gas ejected by the blue hypergiant Wray 977.
Credit: NASA’s Goddard Space Flight Center Conceptual Image Laboratory
GX 301-2 is only about 20 kilometers in diameter. This neutron star rotates on its axis approximately once every 11 minutes. It emits a beam of X-rays that regularly sweeps across Earth, making it a pulsar.
Its gigantic companion plays an essential role in this activity: Wray 977 continuously ejects matter into space in the form of a stellar wind. This is not an ordinary wind, but a stream of very hot gas leaving the star.
During its orbit, the pulsar passes through a region where this gas is particularly dense, and astronomers thought that some of this matter was then captured by the neutron star. This infall would explain the strong X-ray emissions observed over several days.
Artist’s impression of the three phases of GX 301-2’s accretion as it passes through the dense plasma stream from its companion star.
Credit: NASA’s Goddard Space Flight Center Conceptual Image Laboratory
The XRISM space telescope made it possible to directly track the movement of this gas near the pulsar. It observed BP Crucis for about 16 hours on February 1, 2025. Its Resolve instrument analyzes the energy of incoming X-rays with great precision. The researchers notably observed signatures produced by iron; their shift made it possible to measure the speed and direction of the gas.
The matter was moving toward the pulsar at about 540,000 km/h. The observations thus directly confirm that GX 301-2 captures part of its companion star’s wind.
However, this matter does not always fall in the same way. When the pulsar enters the dense region, the captured gas may first begin orbiting it. A turbulent disk then forms before the matter gradually descends toward the neutron star.
Farther along in the gas stream, this disk disappears. The gas can then fall more directly toward the pulsar. This infall releases a large amount of energy: the matter accelerates and heats up considerably as it approaches this extremely dense object.
Some of this energy is emitted as X-rays. The pulsar takes about four days to cross the dense region responsible for this activity.
These measurements provide a better understanding of how a neutron star captures matter from its companion during its orbit. BP Crucis returns to a comparable configuration every 41.5 days. Further observations may show how this gas stream changes from one passage to the next.