Universe Today
Using data from the X-ray Imaging and Spectroscopy Mission (XRISM) observatory, scientists recently observed BP Crucis, a high-mass X-ray binary located about 13,000 light-years away in the southern constellation Crux. The system's primary, Wray 977, is a blue hypergiant of about 40 solar masses and so large and massive that ionized gas constantly streams away from it. Its companion is the tiny neutron star GX 301-2, the pulsar that sweeps its X-ray beam toward Earth with a period of 11 minutes.
This process, where Wray 977 constantly feeds its smaller companion with ionized gas (or "stellar wind"), causes GX 301-2 to release strong X-ray flares. While astronomers have studied X-ray binaries for a long time, this is the first time they have directly observed a giant star's outflow being captured by its compact companion. These studies allow astronomers to better understand some of the most extreme phenomena in the Universe.
The observations were conducted by researchers from the Astrophysics Science Division at NASA's Goddard Space Flight Center, the Center for Space Science and Technology (CSST), the Manipal Centre for Natural Sciences (MCRS), the Israel Institute of Technology, the US Naval Academy, the Lawrence Livermore National Laboratory (LLNL), and multiple universities. A paper describing their findings was published in the journal Science Advances.
The researchers observed this system with XRISM on Feb. 1st, 2025, near the end of one of its stronger flares using the observatory’s Resolve instrument. Over 16 hours, the instrument captured highly detailed X-ray spectra and rapidly changing absorption lines, such as highly ionized iron. This data clearly revealed the speed and direction of plasma relatively close to the pulsar, something astronomers have never witnessed directly before. Said former Goddard researcher Nazma Islam, now an assistant professor at the MCRS in India and a co-author on the paper:
It was clear that these observations were groundbreaking, but at the same time this meant the analysis had to be especially detailed. We could see how the dense stream of plasma acts very close to the neutron star.
Their results show that the observed absorption lines were shifted to lower energies (redshifted), indicating that the ionized gas was receding relative to the observer. In addition to confirming that the stellar wind was flowing toward the pulsar, the observed redshift indicated a velocity of about 540,000 km/h (335,000 mph). From this, the team confirmed something astronomers have long suspected about pulsar binaries.
According to this widely held theory, when a pulsar enters a stream of ionized gas, it sweeps it into a thick, turbulent disk. Like accretion disks around supermassive black holes (SMBHs), this gas spirals toward the pulsar, is heated to extreme temperatures, and emits powerful X-ray flares. As the pulsar moves farther into the stream, the disk breaks down because the stream no longer has enough angular momentum to maintain it.
Once the disk disappears, plasma flows directly onto the neutron star, which the XRISM observations coincided with. As the pulsar nears the end of the stream, a disk briefly returns but spins in the opposite direction and similarly disappears as the pulsar exits the stream. Said Brian Williams, the mission’s project scientist at NASA Goddard:
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.
Further Reading: NASA
