On May 27, 2024, the Wide-field X-ray Telescope (WXT) onboard Einstein Probe detected an unexpected X-ray emission from the Small Magellanic Cloud (SMC), a nearby galaxy. The event, designated EP J0052, was further investigated using multiple space telescopes, including NASA’s Swift and NICER observatories, as well as ESA’s XMM-Newton, which conducted follow-up observations 18 days later.
“We were chasing fleeting sources, when we came across this new spot of X-ray light in the SMC. We realised that we were looking at something unusual, that only Einstein Probe could catch,” said Alessio Marino, a postdoctoral researcher at the Institute of Space Sciences (ICE-CSIC) in Spain and lead author of the study.
The Einstein Probe mission, led by the Chinese Academy of Sciences (CAS), is a collaborative effort with the European Space Agency (ESA), Germany’s Max Planck Institute for Extraterrestrial Physics (MPE), and France’s National Centre for Space Studies (CNES).
Launched on January 9, 2024, from the Xichang Satellite Launch Centre in China, the satellite is equipped with two key instruments: the Wide-field X-ray Telescope (WXT), which continuously scans a vast portion of the sky for unexpected X-ray sources, and the Follow-up X-ray Telescope (FXT), designed to investigate these sources in greater detail.
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Unveiling a rare binary system
Initially, researchers considered the possibility that EP J0052 was a known type of X-ray binary system, typically involving a neutron star accreting matter from a companion. However, further analysis of the X-ray spectrum revealed distinct features suggesting a different origin.
By studying the evolution of the X-ray emission over six days, scientists identified elements such as nitrogen, oxygen, and neon in the ejected material. This led to the conclusion that EP J0052 was not a neutron star system, but rather a rare Be-white dwarf binary.
“We soon understood that we were dealing with a rare discovery of a very elusive celestial couple,” explained Marino. The system consists of a Be-type star, about 12 times the mass of the Sun, and a white dwarf, a compact remnant with a mass similar to the Sun.
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The X-ray flare and the evolutionary pathway
The Be star and the white dwarf orbit closely, with the latter’s strong gravitational pull stripping hydrogen from its massive companion. As this material accumulates on the surface of the white dwarf, the pressure builds until a thermonuclear explosion occurs, generating a sudden burst of radiation across multiple wavelengths, including visible light, ultraviolet, and X-rays.
This process raises questions about the system’s evolution. Be stars typically have short lifespans of around 20 million years, whereas white dwarfs are remnants of stars that lived for billions of years. Given that binary stars usually form together, researchers sought to determine how a white dwarf and a Be star could coexist in the same system.
Scientists propose that the system originally consisted of two large stars, with one slightly more massive than the other. As the larger star exhausted its nuclear fuel, it expanded and transferred material to its companion. This process led to the formation of a gaseous envelope around both stars, which eventually dissipated.
By the end of this phase, the more massive star had shed its outer layers, collapsing into a white dwarf, while the other star grew in mass to become the Be star observed today. Now, the white dwarf is accreting material from its evolved companion, setting the stage for recurrent explosive events.
“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, a research fellow and X-ray astronomer at ESA.
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Implications for stellar evolution research
The follow-up observations with ESA’s XMM-Newton revealed that the X-ray flare had faded completely 18 days after its initial detection, confirming the short-lived nature of such outbursts.
The characteristics of the flare, including the presence of oxygen and neon, suggest that the white dwarf in this system is relatively massive—around 20% more massive than the Sun. Its mass is close to the Chandrasekhar limit, the threshold beyond which a white dwarf would either collapse into a neutron star or explode as a supernova.
“Outbursts from a Be-white dwarf duo have been extraordinarily hard to catch, as they are best observed with low-energy X-rays,” said Erik Kuulkers, ESA’s Project Scientist for Einstein Probe. “The advent of Einstein Probe offers the unique chance to spot these fleeting sources and test our understanding of how massive stars evolve.”
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