A dead star 730 light-years away is pushing a shock wave it cannot power

A white dwarf catalogued as RXJ0528+2838, about 730 light-years from Earth, is surrounded by a curved arc of glowing gas roughly 3,800 times the distance between Earth and the Sun. Astronomers who mapped the arc say it is a bow shock that has been building for at least 1,000 years, and that the star’s present-day magnetic field can account for only a fraction of that history.

The mismatch keeps the object in circulation. ESO announced the discovery in January, and a republication of the release on September 24 put it back in front of readers. The measurements have not changed, and neither has the puzzle.

A bow shock with no disc behind it

A bow shock forms the way a wave piles up ahead of a ship. Material streaming away from a star slams into the thin gas between the stars, and the pile-up glows. The brighter and more extended the glow, the more energy the star must be supplying to keep the outflow running. Around white dwarfs that are feeding on a companion, the usual engine for such an outflow is an accretion disc, a flattened whirlpool of hot gas that spirals inward and flings some of it back out. In its release, the European Southern Observatory describes RXJ0528+2838 as a white dwarf paired with a Sun-like companion, and as a system in which no such disc exists.

Krystian Iłkiewicz, a postdoctoral researcher at the Nicolaus Copernicus Astronomical Center in Warsaw and one of the two lead authors, put the problem bluntly. The observations reveal a powerful outflow that, by current understanding, should not be there. His co-lead, Simone Scaringi of Durham University, called the result something never seen before and, more importantly, entirely unexpected.

The MUSE maps of the nebula

The data came from MUSE, an integral-field spectrograph on ESO’s Very Large Telescope in Chile. An integral-field instrument records a full spectrum at every point of an image, so the team could see not only where the gas sits but what it is made of and how it moves. Those maps tie the nebula to the white dwarf itself instead of to an unrelated cloud in the same patch of sky, and they trace its shape back to a source that has been blowing material outward for centuries.

The work appeared in Nature Astronomy. Durham University’s own account notes that the collaboration spans 12 institutions across seven countries, and ESO’s author list includes Noel Castro Segura of the University of Warwick alongside the two leads.

An energy budget that does not add up

With no disc, the leading idea is that a strong magnetic field on the white dwarf grabs gas from the companion and funnels it straight onto the stellar surface. Durham classes RXJ0528+2838 as a polar white dwarf, the type in which that happens. The trouble is the arithmetic. Durham’s summary of the paper says the star’s current magnetic field is only marginally strong enough to power a bow shock lasting a few hundred years at most, while the shape and size of the structure indicate it has persisted for at least 1,000 years, a figure ScienceDaily’s republication of the ESO text repeats.

That leaves a gap of several centuries in which something kept the outflow going. Either the white dwarf was far more active in the past than it is now, or a second energy source operates that current models leave out. Sci.News, summarising the paper, reports that the known field strength only partly explains what astronomers are seeing, and that a hidden energy source remains to be investigated. Neither explanation has been tested, and the authors do not claim to have settled it.

Looking for a second example

One object cannot say whether the behaviour is rare or merely overlooked. The Durham-led team is searching for similar systems, to learn whether RXJ0528+2838 is a one-off or the first member of a class that earlier surveys passed over because nobody thought to look for faint nebulae around diskless white dwarfs. Scaringi told ESO that even without a disc, these systems can drive powerful outflows, which is the claim the next survey has to test. Science in Poland’s coverage of the team adds that the researchers expect ESO’s planned Extremely Large Telescope to help identify more systems of this kind, since a larger mirror would pick out fainter shells of gas around dimmer, more distant white dwarfs than the Very Large Telescope can reach today.

If a second bow shock turns up around another polar, the energy shortfall would stop looking like a quirk of one star and start looking like missing physics. Until then the paper leaves two numbers for any future model to reconcile: a bow shock at least 1,000 years old, and a magnetic field that can sustain one for only a few hundred.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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