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Newly Discovered Celestial Pair Reveals Insights into Stellar Evolution

Astronomers have identified a unique binary star system comprising a large, hot star over ten times more massive than our Sun and a compact white dwarf with a mass comparable to our star. This intriguing combination is one of only a few similar systems detected to date. For the first time, scientists have been able to monitor the X-ray emissions from this unusual pair from the moment of its sudden flare-up until its luminosity began to wane.

On May 27, 2024, the Wide-field X-ray Telescope (WXT) aboard the Einstein Probe detected a burst of X-rays emanating from our neighboring galaxy, the Small Magellanic Cloud (SMC). This new cosmic source, designated EP J0052, prompted researchers to direct the Follow-up X-ray Telescope of the Einstein Probe towards its location to investigate further.

The identification of this X-ray source also initiated observations by NASA’s Swift and NICER X-ray telescopes, and the European Space Agency’s XMM-Newton telescope followed suit, making observations 18 days later.

“In our pursuit of transient sources, we stumbled upon this X-ray light in the SMC, realizing we were observing something quite exceptional, detectable only by the Einstein Probe,” stated Alessio Marino, a postdoctoral researcher at the Institute of Space Sciences (ICE-CSIC) in Spain and the lead author of the recently published study.

“The WXT distinguishes itself among current X-ray telescopes by its unique ability to detect low-energy X-rays with the sensitivity required to observe this novel source,” Marino added.

Initially, researchers speculated that EP J0052 might belong to a known category of binary systems characterized by X-ray emissions, such as those involving neutron stars regularly siphoning material from massive star companions. However, the collected data suggested a more complex narrative.

A Rare Discovery

Thanks to the timely capture of the X-ray outburst by the Einstein Probe, scientists conducted extensive analysis using data from various instruments. They scrutinized variations in light across different X-ray wavelengths over a period of six days, revealing elements present in the ejected material, including nitrogen, oxygen, and neon. This analysis provided significant clues about the nature of the system.

“We quickly realized we were witnessing a rare finding of a very elusive celestial pair,” explained Marino. “This peculiar duo includes a massive star, referred to as a Be star, with a mass twelve times that of the Sun, and a compact white dwarf, which is extraordinarily dense and has a mass comparable to that of our Sun.”

The two stars are engaged in a close orbital relationship, where the white dwarf’s powerful gravitational force pulls material from its companion star. As the white dwarf accumulates hydrogen from the Be star, the deposited material undergoes compression until it triggers a runaway nuclear fusion reaction, resulting in a brilliant flash across a spectrum of wavelengths, from visible light to ultraviolet and X-ray emissions.

At first glance, the existence of this binary system raises questions, as Be stars typically consume their nuclear fuel rapidly, with lifecycles lasting around 20 million years. Conversely, the white dwarf, which is generally the remnants of a star similar to our Sun, would conventionally have a much longer lifespan stretching over several billion years.

How, then, does a short-lived star exist harmoniously alongside a star that should have long since ceased to shine?

There is a plausible explanation.

A Tale of Two Stars

Astrophysicists theorize that this binary system began as a more balanced pair of massive stars, one six and the other eight times the mass of the Sun. The larger star consumed its nuclear fuel more rapidly, leading it to expand and shed material onto its companion. Initially, the Be star’s thick outer layers were consumed, followed by the ejection of its remaining shells, creating an envelope around the two stars that eventually formed a disc and dissipated.

By the culmination of this stellar drama, the companion star had transformed into a Be star with a mass of twelve solar masses, while the remnants of the original larger star collapsed into a white dwarf weighing just over one solar mass. The cycle continues, with the white dwarf now drawing material from the Be star’s outer layers.

“Our study offers fresh perspectives on a seldom-observed phase of stellar evolution resulting from a complex material exchange between the two stars,” noted Ashley Chrimes, a research fellow and X-ray astronomer at ESA. “It is indeed captivating to witness how an interacting pair of massive stars can yield such an extraordinary outcome.”

ESA’s follow-up observation with the XMM-Newton mission, which occurred 18 days after the initial detection by the Einstein Probe, showed that the X-ray signal had vanished. This absence places a temporal limit on the flare duration, suggesting it was relatively brief.

The short-lived nature of the outburst, combined with the detection of neon and oxygen, indicates that the white dwarf is likely of a heavier variety, possibly about 20% more massive than the Sun. This mass approaches the Chandrasekhar limit, the threshold beyond which it could either continue to collapse into a neutron star or erupt in a supernova event.

Game-Changing Monitor

“Detecting outbursts from a Be-white dwarf binary pair has proven exceptionally difficult, as optimal observation conditions rely on low-energy X-rays. The advent of the Einstein Probe provides a unique opportunity to identify these ephemeral sources and refine our understanding of massive star evolution,” commented Erik Kuulkers, ESA Project Scientist for the Einstein Probe.

“This discovery highlights the groundbreaking capabilities of our mission.”

About Einstein Probe

The Einstein Probe represents a collaborative mission involving the Chinese Academy of Sciences (CAS), the European Space Agency (ESA), Germany’s Max-Planck Institute for Extraterrestrial Physics (MPE), and France’s Centre National d’Études Spatiales (CNES). Launched on January 9, 2024, from the Xichang Satellite Launch Centre in China, the probe is equipped with two instruments: the Wide-field X-ray Telescope (WXT), which continuously surveys a substantial portion of the sky for unexpected X-ray emissions, and the Follow-up X-ray Telescope (FXT), which focuses on the identified X-ray sources for detailed analysis.

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