Astronomers using the XRISM space observatory have observed plasma from a stellar wind falling onto a pulsar at 540,000 kilometres per hour for the first time.
The high-speed accretion event was recorded in BP Crucis, a binary star system located 13,000 light-years from Earth. Measurements revealed the compact neutron star GX 301-2 drawing in ionized gas stripped from its massive companion star, Wray 977.
Wray 977 is a blue hypergiant star with a mass roughly 40 times that of the Sun and a diameter about 60 solar diameters across. It continuously sheds a powerful wind of ionized gas into surrounding space. Its orbiting companion, GX 301-2, is a dense neutron star with a mass greater than the Sun, packed into a sphere roughly 20 kilometres in diameter that rotates once every 11 minutes.
Twice during its 41.5-day orbit around the hypergiant, the pulsar passes through an exceptionally dense stream of stellar material, triggering intense X-ray flares in the system. On 1 February 2025, the XRISM observatory monitored the system for approximately 16 hours during one of these flare episodes.
Spectral lines reveal plasma movement
During the 16-hour observation window, the Resolve spectrometer aboard XRISM detected absorption lines from highly ionized iron atoms. These spectral lines were distinctly shifted toward lower energy levels. The observed redshift proved that the plasma was moving directly toward the pulsar, while the magnitude of the shift allowed researchers to calculate its speed at about 540,000 kilometres per hour.
XRISM, short for the X-ray Imaging and Spectroscopy Mission, is a joint space astronomy project led by the Japan Aerospace Exploration Agency in collaboration with NASA. Launched to study high-energy radiation, the observatory uses advanced X-ray spectroscopy to analyze celestial phenomena, including gas flows around compact cosmic bodies.
Neutron stars are extremely dense stellar remnants formed when massive stars collapse at the end of their lifecycle. When a neutron star possesses a strong magnetic field and rapidly rotates, it acts as a pulsar, emitting beams of electromagnetic radiation across space.
Gas disk behavior during flare passage
Astrophysicists believe that as the pulsar enters the dense stellar wind stream, the captured gas forms a turbulent disk around the neutron star. The plasma gradually spirals inward, heating up and emitting strong X-ray radiation before the disk breaks down as it moves through the stream.
When the disk disintegrates, the plasma begins falling directly onto the neutron star. Shortly before exiting the gas stream, the disk briefly reforms, but with its rotation reversed in the opposite direction.
The complete transition through the dense gas stream takes about four days. The observations mark the first time scientists have traced in detail how material from stellar wind transforms into the driving energy source for space X-ray flares.
