A binary star system called BP Crucis, located about 13,000 light-years from Earth, consists of a massive blue hypergiant star named Wray 977, which is about 60 times larger than the sun, and a neutron star called GX 301-2. Neutron stars are extremely dense, rapidly rotating remnants of exploded stars, and some of them, called pulsars, emit beams of electromagnetic radiation, such as X-rays, at regular intervals. When these beams point toward Earth, telescopes detect brief flashes of light. GX 301-2 is unusual because, in addition to its regular flashes, it also produces frequent, intense X-ray flares that make it shine much brighter at repeated intervals. Scientists had previously been puzzled by the cause of these flares. A study published Sept. 18 in the journal Science Advances used the X-ray Imaging and Spectroscopy Mission (XRISM), a space telescope jointly operated by NASA and the Japan Aerospace Exploration Agency, to investigate BP Crucis. The researchers found that the neutron star flares up every time it passes through a massive plume of "stellar wind plasma" — a stream of charged particles — emitted by its massive companion star, Wray 977. This discovery marks the first time scientists have observed such stellar material interacting with a neutron star or similar stellar remnant. GX 301-2 orbits Wray 977 every 41.5 days, and its X-ray flares occur when it reaches its closest and farthest points in its orbit. The strongest flares happen at the closest point, where the neutron star encounters the densest part of the stellar wind plume. Scientists had long suspected that the flares were caused by the neutron star passing through this plume, but they lacked direct evidence until now. During a 16-hour observation period that coincided with one of the neutron star's flares, researchers used XRISM to capture highly detailed X-ray spectra. These spectra revealed rapidly changing emission and absorption lines that showed how the plume moved and changed around GX 301-2. By analyzing this data, the researchers were able to simulate how the plasma from the stellar wind interacts with the neutron star. When enough material accumulates on the neutron star's surface, it triggers an explosion, resulting in the bright flashes detected on Earth. The team now hopes to use XRISM to study future flaring events and better understand the dynamic relationship between GX 301-2 and the windy plume from Wray 977.