China Focus: China's Einstein Probe captures rare X-ray flash from binary star system

BEIJING -- China's Einstein Probe (EP) astronomical satellite has captured an X-ray flash from a rare and elusive binary star system, offering new insights into the interaction and evolution of massive stars.

The research, a collaboration between Chinese and international scientists, was published in the latest issue of The Astrophysical Journal Letters.

The binary system consists of a large, hot star 12 times the mass of the Sun, and a compact white dwarf with a mass similar to that of the Sun but only the size of the Earth. Only a handful of such systems have been identified, and this is the first time scientists have tracked the X-ray light from the pair as it flared up and then faded.

On May 27, 2024, the Wide-field X-ray Telescope (WXT) onboard the EP satellite detected X-rays from the Small Magellanic Cloud, a neighboring galaxy. To trace the source, identified as EP J0052, scientists used EP's Follow-up X-ray Telescope (FXT) and also enlisted NASA's Swift and NICER X-ray telescopes, as well as the European Space Agency (ESA)'s XMM-Newton telescope.

Data analysis revealed the source to be a rare and intriguing celestial pair.

"We realized that we were looking at something unusual, that only EP could catch. This is because, among current telescopes monitoring the X-ray sky, WXT is the only one that can see lower energy X-rays with sufficient sensitivity to catch the novel source," says Alessio Marino, a researcher at the Institute of Space Sciences in Spain, and lead author of the study.

"The unusual duo consists of a massive star that we call a 'Be star,' weighting 12 times the Sun, and a stellar 'corpse' known as a white dwarf, a compact and hyper-dense object, with a mass similar to that of our star," explains Marino.

The two stars orbit closely, with the white dwarf's strong gravitational field pulling material from its companion. This process eventually leads to a catastrophic nuclear explosion, creating a bright flash across multiple wavelengths, including visible light, UV and X-rays.

According to the scientists, the two stars' interaction began with the larger star exhausting its nuclear fuel, shedding material onto its companion. As the Be star grew to 12 times the mass of the Sun, the remaining core of the other star collapsed into a white dwarf. Now, the white dwarf is pulling material from the Be star's outer layers.

"This study gives us new insights into a rarely observed phase of stellar evolution, which is the result of a complex exchange of material that must have happened among the two stars," said Ashley Chrimes, an X-ray astronomer at ESA. "It's fascinating to see how an interacting pair of massive stars can produce such an intriguing outcome."

Erik Kuulkers, ESA project scientist for EP, noted that outbursts from Be-white dwarf systems are extraordinarily difficult to observe. "The advent of EP offers the unique chance to spot these fleeting sources and test our understanding of how massive stars evolve."

The EP mission is one of a series of space science missions led by the Chinese Academy of Sciences. It is also an international collaboration mission with contributions from the ESA, the Max Planck Institute for Extraterrestrial Physics in Germany, and the French space agency CNES.

Launched on Jan 9, 2024, from Xichang Satellite Launch Center in Sichuan province, Southwest China, the EP satellite carries two scientific instruments: the WXT, which provides a wide view of the X-ray sky, and the FXT, which allows for detailed observation of transient sources detected by the WXT.

EP is an international collaborative mission, and its science team comprises about 300 researchers worldwide. The recent publication of the first paper led by scientists from the ESA member states based on EP data highlights the project's openness and collaborative spirit in scientific research, said Yuan Weimin, EP's principal investigator.

"We hope that the EP satellite will continue to provide invaluable observational datasets for the worldwide astronomical community, driving advancements in humanity's understanding of the ever-changing universe," he added.