XRISM Makes First-Ever Detection of a Pulsar Feeding from a Companion's Stellar "Wind"

The XRISM observatory has made history by detecting a pulsar feeding from a companion star's stellar wind for the very first time. The system, known as BP Crucis, resides approximately 13,000 light-years away in the southern constellation Crux. Its massive blue hypergiant primary star, Wray 977, constantly releases ionized gas into space. Accompanying this giant is a neutron star named GX 301-2, which serves as the pulsar, emitting an X-ray beam that periodically sweeps toward Earth.

This unique scenario, where a blue hypergiant star feeds its smaller companion with stellar wind, allowed scientists to observe an unprecedented phenomenon. The observed absorption lines shifted towards lower energies (redshifted), indicating that the ionized gas was moving away from the observer. Crucially, the redshift provided a velocity of around 540,000 km/h (335,000 mph), confirming that the stellar wind was indeed flowing towards the pulsar.

These findings provided the first direct evidence that a giant star's outflow can be captured by its compact companion.

Researchers from various institutions, including NASA's Goddard Space Flight Center, the Center for Space Science and Technology, the Manipal Centre for Natural Sciences, the Israel Institute of Technology, the US Naval Academy, the Lawrence Livermore National Laboratory, and multiple universities, collaborated on the study. The data was collected on February 1, 2025, during one of the system's stronger flares using XRISM's Resolve instrument.

Over the course of 16 hours, the instrument captured highly detailed X-ray spectra and rapidly changing absorption lines, such as highly ionized iron.

According to widely accepted theory, when a pulsar enters a stream of ionized gas, it creates a dense, turbulent disk that spirals towards the pulsar, heating up to extreme temperatures and emitting powerful X-ray flares. As the pulsar moves deeper into the stream, the disk breaks down due to the lack of angular momentum in the gas.

Once the disk disappears, plasma flows directly onto the neutron star. In the case of BP Crucis, the XRISM observations coincided with this transition. As the pulsar nears the end of the stream, a disk briefly reappears but spins in the opposite direction before disappearing as the pulsar exits the stream.

Dr. Nazma Islam, a co-author and former Goddard researcher now at the Manipal Centre for Natural Sciences, emphasized the groundbreaking nature of these observations, noting that the analysis had to be particularly meticulous to capture the fine details of the plasma's behavior near the neutron star. The results of this research, published in the journal Science Advances, offer valuable insights into some of the most extreme phenomena in the Universe, as noted by Dr. Brian Williams, the mission's project scientist at NASA Goddard.