Observations by the Very Large Telescope in Chile reveal a “bow shock wave” nebula around a seemingly quiet, diskless binary system, hinting at a magnetic “engine” that is still not understood
Streams of gas and dust emitted from stars can, under the right conditions, collide with the environment surrounding the star and create shock wave. Now, astronomers using ESO's Very Large Telescope (VLT) have captured a beautiful shock wave around a stellar remnant — a discovery that has left them baffled. By all currently known mechanisms, the small remnant RXJ0528+2838 should not be surrounded by such a structure. The discovery, which is as mysterious as it is spectacular, challenges our understanding of how stellar remnants interact with their environment.
“We found something that has never been seen before, and more importantly, something that we didn’t expect at all,” says Simon Scaringi, an associate professor at Durham University (UK) and lead co-author of the study, published today in the journal Nature Astronomy. “Our observations reveal a powerful outflow that, according to our current understanding, should not exist,” adds Krzysztof Ilkiewicz, a postdoctoral fellow at the Nicolaus Copernicus Astronomical Centre in Warsaw (Poland), also a lead co-author. In astronomy, an “outflow” is a term for material ejected from celestial objects into space.
Arc shock wave
The system RXJ0528+2838 is about 730 light-years away. Like the Sun and other stars, it orbits the center of the galaxy. As it moves, it interacts with the gas that fills the space between the stars, creating a “bow shock” — “a curved arc of material, like a wave built up at the bow of a ship,” explains Noel Castro Segura, a research associate at the University of Warwick (UK) and co-author of the study. Such shock waves are usually created by material ejected from the central star, but in the case of RXJ0528+2838, none of the known mechanisms fully explain what was observed.
RXJ0528+2838 is white dwarf — the core left over from a low-mass star that has ended its life — and a Sun-like companion orbiting it. In such binary systems, material from the companion star can pass into the white dwarf, often forming a halo around it. diskWhile the disk “feeds” the remnant, some of the material is also ejected into space, creating strong outflows. However, there is no sign of a disk in RXJ0528+2838, so the source of the outflow—and the accompanying nebula around the star—remains a mystery.
“The surprise that a seemingly quiet system, without a disk, could drive such an impressive nebula was one of those rare ‘wow’ moments,” says Scaringi.
The team first noticed the unusual nebula around RXJ0528+2838 in images from the Isaac Newton Telescope in Spain. Having identified the unusual shape, they observed it in more detail using the MUSE instrument on ESO’s Very Large Telescope. “The observations with MUSE allowed us to map the rainbow shock wave in detail and analyse its composition. This was crucial to confirm that the structure did indeed originate in the binary system, and not in another unrelated nebula or interstellar cloud,” explains Ilkiewicz.
The shape and size of the shock wave suggest that the white dwarf has been emitting a powerful outflow for at least about 1000 years. Scientists still don't know how a diskless system can sustain such a sustained outflow—but they do have a possible direction.
This white dwarf is known to have Strong magnetic field, and this has also been confirmed by MUSE data. Such a field could funnel material “stolen” from the companion star directly to the white dwarf, without a disk forming around it. “Our finding shows that even without a disk, such systems can produce strong outflows—revealing a mechanism that we still don’t understand. The discovery challenges the standard picture of how matter moves and interacts in such extreme binary systems,” says Ilkiewicz.
The findings hint at a hidden energy source—most likely the strong magnetic field—but this “mysterious engine,” as Scaringi calls it, still requires investigation. The data suggest that the current magnetic field is strong enough to sustain a bow shock wave for only a few hundred years, and therefore explains only part of the observed phenomenon.
To better understand the nature of outflows in discless systems, many more binary systems will need to be studied. ESO’s future giant telescope, the Extremely Large Telescope (ELT), is expected to help with this: it will allow astronomers to map more systems — even fainter ones — and locate similar systems in detail, ultimately getting closer to understanding the mysterious source of energy that remains unexplained, Scaringi believes.
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