Einstein Probe catches X-ray odd couple

The odd celestial couple consists of a big, hot star, more than 10 times larger than our Sun, and a small compact white dwarf, with a mass similar to our star. Only a handful of these systems have been found so far. And this the first time scientists could track the X-ray light coming from such a curious pair from its initial sudden flare-up to its fading away.

On 27 May 2024, the Wide-field X-ray Telescope (WXT) on Einstein Probe spotted X-rays coming from within our neighbour galaxy, the Small Magellanic Cloud (SMC). To uncover the origin of this new celestial beacon, labelled EP J0052, scientists pointed Einstein Probes’s Follow-up X-ray Telescope in that direction.

WXT’s observations also triggered NASA’s Swift and NICER X-ray telescopes to point to the newly discovered object. ESA’s XMM-Newton followed up 18 days after the trigger.

“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,” says Alessio Marino, a postdoctoral researcher at the Institute of Space Sciences (ICE-CSIC), Spain and lead author of the new study published today.

“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.”

Initially, the scientists thought EP J0052 might be a well-known type of binary system that shines in X-rays. These pairs consist of a neutron star devouring up material from a massive star companion. Yet, there was something in the data telling a different story…

A rare discovery

Thanks to Einstein Probe catching the novel source right from its initial flash, scientists could analyse batches of data from different instruments. They examined how the light varied across a range of X-ray wavelengths, over six days, and teased out some of the elements present in the exploding material, such as nitrogen, oxygen and neon. The analysis delivered crucial clues.

“We soon understood that we were dealing with a rare discovery of a very elusive celestial couple” explains Alessio. “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.”

The two stars closely orbit each other, and the white dwarf’s intense gravitational field pulls matter from its companion. As more and more material (mainly hydrogen) rains down on the compact object, its strong gravitation compresses it, until a runaway nuclear explosion is initiated. This creates a bright flash of light across a wide range of wavelengths from visible light to UVs and X-rays.

At first sight, the existence of this duo is puzzling. Massive stars of type Be burn fast through their reserve of nuclear fuel. Their lives are fierce and short, spanning about 20 million years. Its companion is (usually) the collapsed remnant of a star similar to our Sun that in isolation would live for several billions of years.

Since binary stars typically form together, how can the supposedly short-lived star still be shining bright, while the alleged long-lived one has already died?

There is an explanation.

A tale of two stars

Scientists think that the couple started off together, as a better-matched binary pair consisting of two rather big stars, six and eight times more massive than our Sun.

The bigger star exhausted its nuclear fuel earlier and started to expand, shedding matter to its companion. First, gas in its puffed-up outer layers got pulled in by the companion; then its remaining outer shells got ejected, forming an envelop around the two stars, which later became a disc, and finally dissolved.

By the end of this drama, the companion star had grown to be 12 times the mass of the Sun, while the outstripped core of the other had collapsed to become a white dwarf of just over one solar mass. Now, it is the turn of the white dwarf to steal and gobble up material from the outer layers of the Be star.

“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,” remarks 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.”

ESA’s XMM-Newton mission’s follow-up observation in the direction of EP J0052, 18 days after Einstein Probe’s first look, did not see the signal anymore. This sets a limit on the duration of the flare, showing it to be relatively brief.

The duration of the short burst, and the presence of neon and oxygen, hint at a rather heavy type of white dwarf, likely 20% more massive than the Sun. Its mass is close to the level, called Chandrasekhar limit, above which the star would continue to implode, and become an even denser neutron star, or explode as a supernova.

Game-changing monitor

“Outbursts from a Be-white dwarf duo have been extraordinarily hard to catch, as they are best observed with low energy X-rays. The advent of Einstein Probe offers the unique chance to spot these fleeting sources and test our understanding of how massive stars evolve,” remarks Erik Kuulkers, ESA Project Scientist for Einstein Probe.

“This discovery showcases the game-changing capabilities of this mission.”

About Einstein Probe

Einstein Probe is a mission of the Chinese Academy of Science (CAS) working in partnership with the European Space Agency (ESA), the Max-Planck-Institute for extraterrestrial Physics (MPE), Germany, and the Centre National d'Études Spatiales (CNES), France. It was launched from the Xichang Satellite Launch Centre in China on 9 January 2024, and carries two instruments. The Wide-field X-ray Telescope (WXT) constantly monitors a large portion of the sky for unexpected X-rays, and the Follow-up X-ray Telescope (FXT) that homes in on the X-ray sources found by WXT for a more detailed look.