A rare X-ray flash emanating from a unique binary star system was recently spotted by the Einstein Probe, treating an international team of astronomers to a rare celestial occurrence.
The probe’s latest discovery offers essential new data on the evolution of massive stars and how they interact, as well as the Einstein probe’s formidable capabilities in spotting transient X-ray sources from deep in space.
The Einstein probe, led by the Chinese Academy of Sciences (CAS) in partnership with the European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics (MPE), is focused on collecting data on high-energy astrophysics. Since its launch in January 2024, the probe has worked toward discovering powerful transients and other variable celestial objects.
Einstein Probe Spots an Uncommon Event
The celestial event, which originated from a region within the Small Magellanic Cloud (SMC), involved a pairing between a massive Be-type star roughly 12 times the size of the Sun and a smaller, much denser white dwarf.
The X-ray burst was revealed as it suddenly flared and slowly subsided, allowing the first real-time tracking of such an event.
The unexpected X-ray flash, later designated EP J0052, was observed with Einstein’s Wide-field X-ray Telescope (WXT) on May 27, 2024. Additional observations made by other space observatories, including NASA’s Swift and NICER and ESA’s XMM-Newton, produced further data on the object just weeks later.
“We were chasing fleeting sources when we came across this new spot of X-ray light,” said lead author Alessio Marino, a postdoctoral researcher at the Institute of Space Sciences (ICE-CSIC) in Spain. “We realized we were looking at something unusual that only Einstein Probe could catch.”
An Atypical X-Ray Burst
Unlike most binary systems that emit X-rays, which sometimes involve neutron stars that drag material away from their nearby companions, EP J0052 revealed something new to astronomers.
Over the course of several days, the research team analyzing EP J0052 identified key elements in the material ejected when the flare was produced, including nitrogen, neon, and oxygen, revealing important clues to the underlying nature of the system.
Based on additional analysis, the team was able to verify that EP J0052 consists of an extremely rare pairing of a Be-type star and a white dwarf companion subjected to intense gravitational forces. The immensity of the white dwarf’s gravitational influence draws hydrogen-rich material away from its massive companion, which, over time, accumulates and triggers a runaway nuclear explosion.
The resulting flash of the nuclear event generates illumination across the electromagnetic spectrum, ranging from visible light to X-rays.
An Astronomical Puzzle
Since Be stars burn off their nuclear power stores very quickly, they typically only last for around 20 million years—a relatively short period in astronomical terms. By comparison, a white dwarf represents the remains left behind by a star that once resembled the Sun, meaning it would require billions of years to reach this stage.
Since binary stars normally form together, the binary system spotted by Einstein is puzzling to astronomers. It remains unknown how one star could already be a stellar remnant while its companion is still in its prime.
One possible explanation is that the system began as a more balanced binary pair, at which time two large stars—one roughly six times the mass of the Sun and the other at around eight solar masses—were companions. However, with time, the larger of the two stars exhausted its fuel, ultimately expanding its companion as the material was transferred to it.
As the initially larger star was stripped down to its core, eventually becoming the white dwarf astronomers see today, the surviving companion grew until it reached its present form as a Be star.
“This study gives us new insights into a rarely observed phase of stellar evolution,” said Ashley Chrimes, research fellow and X-ray astronomer at ESA. “It’s fascinating to see how an interacting pair of massive stars can produce such an intriguing outcome.”
Additional Observations
Further insights were provided by the ESA’s XMMXMM-Newton telescope, which produced follow-up observations revealing EP J0052’s X-ray signal had already faded after 18 days.
The ESA’s observations helped confirm the speed at which the X-ray flash occurred and the duration of the outburst, and elements detected as it took place. According to the additional ESA data, the white dwarf was judged to be unusually massive, close to being 20% heavier than the Sun.
With such a significant mass, the white dwarf exists near what astronomers call the Chandrasekhar limit. This limit describes the threshold beyond which a white dwarf collapses into a neutron star or explodes as a supernova.
Astronomers have long hoped to observe such an event in real-time, but this is challenging since outbursts produced by Be stars and white dwarf binaries are most easily observed when they involve lower-energy X-rays, which many telescopes have difficulty detecting.
“Outbursts from a Be-white dwarf duo have been extraordinarily hard to catch,” said Erik Kuulkers, ESA Project Scientist for Einstein Probe. Fortunately, Einstein’s unique ability to monitor the X-ray sky with such sensitivity offers astronomers an ideal tool, as it is optimized for spotting such fleeting celestial events.
“The advent of Einstein Probe offers the unique chance to spot these fleeting sources and test our understanding of how massive stars evolve,” Kuulkers said.
Fundamentally, EP J0052’s discovery represents one of the best examples to date of the Einstein Probe’s game-changing observational abilities. It offers astronomers new insights into the unique and often violent interactions that take place between stars in binary systems.
The team’s discoveries were detailed in a new paper, “Einstein Probe Discovery of EP J005245.1−722843: A Rare Be–White Dwarf Binary in the Small Magellanic Cloud?” which appeared in The Astrophysical Journal Letters on February 18, 2025.
Additional mission data on the Einstein Probe and its discoveries, as well as its unique “Lobster Eye” optics, can be found at the ESA’s website.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.