A neutron star in a binary system captures ionized gas that continuously flows from its massive companion. The infall of this plasma is accompanied by powerful X-ray flares. Astronomers have directly traced such a process for the first time. The motion of the matter was measured using Japan’s space-based X-ray observatory, created with NASA participation.
Stellar Wind Speed
The BP Crucis system was observed on February 1, 2025, using the X-Ray Imaging and Spectroscopy Mission (XRISM). The observing session lasted 16 hours and coincided with the end of one of the powerful flares.
As NASA reports, detailed X-ray spectra were obtained with the Resolve spectrometer. The instrument detects individual X-ray particles. When each one strikes a tiny absorber, it heats it slightly. The energy of the particle is determined from the amount of this heating. To detect such a tiny change in temperature, the absorber is cooled to nearly absolute zero. This allows the spectrometer to distinguish even very closely spaced spectral lines that previous instruments blended into a single broad band.
Rapidly changing absorption lines appeared in the spectra, including those belonging to highly ionized iron. These lines were shifted toward lower energies. Such a redshift means that the gas was moving away from the observer.
From the magnitude of the shift, researchers directly determined for the first time the speed and direction of matter moving near a neutron star. The stellar wind — the flow of gas from the massive companion — moves toward it at about 540,000 kilometers per hour. At that speed, plasma would cover the straight-line distance from Kyiv to Lviv, about 470 kilometers, in just over three seconds.
Hypergiant and Pulsar
This binary system is located about 13,000 light-years from Earth, according to Universe Today. The system’s primary star, Wray 977, is a blue hypergiant with a mass of about 40 Suns. Because of its enormous size and mass, ionized gas continuously flows from it.
The hypergiant’s companion is the neutron star GX 301-2. It rotates and directs an X-ray beam toward Earth once every 11 minutes, so it is observed as a pulsar.
A Disk Around the Pulsar
The results, published in the peer-reviewed journal Science Advances, confirmed a hypothesis that astronomers had long proposed for such systems. When the pulsar enters the stream of ionized gas, it twists the gas into a thick, turbulent disk similar to the disks around supermassive black holes. Matter in the disk spirals toward the neutron star, heats up, and becomes a source of X-ray radiation.
Deeper within the stream, the gas no longer has enough angular momentum — the reserve of rotational motion — for the disk to continue to exist. Eventually, the disk breaks apart, and the plasma falls directly onto the neutron star. The XRISM observations happened to capture precisely this phase.
Closer to the edge of the stream, the disk briefly appears again, but rotates in the opposite direction from the first one. As soon as the pulsar leaves the stream, the second disk also disappears.
Artist’s illustration of the pulsar in the BP Crucis system passing through a stream of plasma from the blue hypergiant. Credit: NASA Goddard Space Flight Center Conceptual Image Laboratory
A Natural Laboratory
Study co-author Nazma Islam of the Manipal Centre for Natural Sciences in India emphasizes that such data were obtained for the first time, so the analysis required particular care. The team observed a dense stream of plasma moving very close to the neutron star.
XRISM project scientist Brian Williams of NASA’s Goddard Space Flight Center calls BP Crucis an ideal laboratory for studying how matter from stellar winds falls onto pulsars. In his view, the Resolve spectrometer is particularly well suited to such research because of its high sensitivity and resolution.
