The newly identified stellar system consists of a massive Be-type star, over ten times the size of the Sun, and a compact white dwarf with a comparable mass to our star. These pairings are extremely rare, and this marks the first instance where researchers have observed the X-ray signature of such a system from its initial outburst to its gradual fading.
On May 27, 2024, the Einstein Probe's Wide-field X-ray Telescope (WXT) detected an unexpected X-ray signal emanating from the Small Magellanic Cloud (SMC), a neighboring galaxy. The object, labeled EP J0052, quickly drew the attention of astronomers, prompting further observations by the probe's Follow-up X-ray Telescope. Additionally, NASA's Swift and NICER X-ray telescopes, along with ESA's XMM-Newton, joined in to analyze the event over subsequent days.
"We were searching for transient sources when we spotted this new X-ray emission in the SMC. It became clear that only Einstein Probe had the sensitivity to detect such a fleeting and rare phenomenon," explained Alessio Marino, a postdoctoral researcher at the Institute of Space Sciences (ICE-CSIC) in Spain and lead author of the study published this week.
WXT's unique ability to detect low-energy X-rays with high sensitivity enabled scientists to gather extensive data on EP J0052. Initially, the system appeared similar to known binary star configurations, where a neutron star feeds on a massive stellar companion. However, a detailed spectral analysis provided key clues suggesting otherwise.
The probe's early detection allowed researchers to study the system's light curve across different X-ray wavelengths over six days, identifying elements such as nitrogen, oxygen, and neon in the expelled material. These findings confirmed that the system was not a neutron star binary but a white dwarf paired with a massive Be-type star-a rare and little-understood configuration.
"We soon realized we were dealing with an exceptionally rare binary system," Marino stated. "This system comprises a Be star about 12 times the Sun's mass and a white dwarf, a hyper-dense remnant of a once-massive star, with a mass close to that of the Sun."
In this duo, the white dwarf's gravitational pull siphons material from the Be star. As hydrogen accumulates on its surface, it reaches a critical threshold, triggering a thermonuclear explosion that generates a brilliant flash of light spanning visible, ultraviolet, and X-ray wavelengths.
The presence of such a pair presents a puzzling evolutionary scenario. Be stars burn through their fuel rapidly and typically have lifespans of about 20 million years. Meanwhile, white dwarfs originate from stars similar to the Sun, which normally evolve over billions of years before becoming stellar remnants. How then could a supposedly short-lived Be star remain intact while its partner had already died and become a white dwarf?
Astronomers suggest that the system likely formed as a binary with two similarly massive stars-one about six times the Sun's mass and the other about eight times. The larger star would have evolved faster, shedding its outer layers and transferring material to its companion. This process led to the growth of the Be star while the stripped-down core of the first star collapsed into a white dwarf. Now, the white dwarf continues the cycle by accreting material from the Be star, triggering occasional outbursts like the one Einstein Probe observed.
"This study offers valuable insight into a rarely observed stage of stellar evolution, revealing the intricate exchange of mass between binary stars and its consequences," noted Ashley Chrimes, an X-ray astronomer and research fellow at ESA.
The fleeting nature of the X-ray flare became apparent when ESA's XMM-Newton telescope conducted a follow-up observation 18 days after the initial detection. By then, the signal had completely faded, confirming the short-lived nature of the event.
Further analysis revealed that the system's white dwarf is relatively heavy-about 20% more massive than the Sun. It is approaching the Chandrasekhar limit, the maximum mass a white dwarf can sustain before collapsing into a neutron star or triggering a supernova.
"Capturing these outbursts has been incredibly difficult, as they are best detected using low-energy X-rays," said Erik Kuulkers, ESA Project Scientist for Einstein Probe. "Einstein Probe's capabilities have made it possible to track these fleeting sources and refine our understanding of massive star evolution."
With its game-changing ability to detect and analyze transient X-ray phenomena, the Einstein Probe is poised to uncover more such hidden interactions in the universe, furthering our knowledge of stellar evolution and binary star dynamics.
Research Report:Einstein Probe discovery of EP J005245.1-722843: a rare BeWD binary in the Small Magellanic Cloud?
Related Links
Einstein Probe at ESA
Stellar Chemistry, The Universe And All Within It
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